Hawkman and Donna Troy have stories so complicated that they have their personal pages here and here.
Most DC characters (Superman, Batman, Wonder Woman, Green Lantern, The Flash, Hawkman, etc) were created during in the 1930s and 1940s. At that time, a man dressed in the American flag taking down Hitler was everything that readers needed. World War II ended, the interest in such things died down, and most comic books began to close or to move to other genres. In 1956, Gardner Fox and Carmine Infantino created a new Flash, unrelated to the old one in everything except the name (and indeed, confirming in his first appearance that the old one only existed as an in-universe comic book character), and the superhero genre was reborn, followed by similar relaunches of old DC glories. Did you follow up here? Well, one day Fox wanted a cameo appearance of the old Flash, and wrote "Flash Of Two Worlds". Flash (Barry Allen) appears by accident in another world, "Earth 2", where the original Flash lives. They meet, save the day, Barry comes back home, and their meeting opens the Pandora's box. What happens with Superman and Batman, whose titles had never been canceled? Which stories are in Earth 1 and which ones in Earth 2? If Barry knows Jay's secret identity because there are in-universe comics about the Earth 2 characters, what happens with Clark Kent and Bruce Wayne? And why stop it with 2 Earths and not create new ones? All this led to DC deeming that the multiverse had become too "convoluted" and torching the whole thing in Crisis on Infinite Earths, which destroyed all alternate Earths save for the main one, which is not pre-Crisis Earth-1 but a new one that combines several aspects of both (location, characters, backstory, etc). However, it was not followed by a full continuity reboot: some comics were rebooted, and others continued as always, rebooting just specific things they needed. Of course, this leads to several pre and post Crisis things that were equally canon, despite the inconsistencies. Ninety percent of DC's continuity snarls and messed backstories can be traced back to the Crisis and its aftermath.
New 52: There was no real broad plan for what the new relaunched DCU would look like, and nearly all books and characters were completely rebooted... but just enough were kept the same to really create problems. This was mostly due to Green Lantern staying completely canon and Batman keeping about 80% of its prior history, when both books had spent the past few years getting their fingers in just about every pie of the post-Infinite Crisis continuity. What was more, the new continuity lacked any kind of franchise bible, leading to rather poor communication between teams due to nobody being all that clear on what was or wasn't canon (barring what the editorial staff wanted Exiled from Continuity). Consequently, you had many, many, many cases of characters appearing or events being referenced that absolutely could not fit into the New 52 as established.
The New 52 stated that the events of the War of the Light still occurred, but that would also mean that all the events leading up to it are also in continuity including Hal Jordan becoming The Spectre after Jim Corrigan gave it up, and then passing it onto Crispus Allen who helped the character during the various crisises (including subduing the red Butcher entity). In the New 52, the Spectre is once again Jim Corrigan, and has never stated to be any one else. The Alan Scott Green Lantern, who acted in a mentoring role to Kyle Rayner and is now located on a different Earth, which means that he now never helped during the Blackest Night or the Power of Ion storyline. Plastic Man first appears in Forever Evil (2013), which also has Sinestro in his post Parallax-inspired Wrath of the First Lantern costume, which means it takes place after those events, but Plastic Man was present with Kyle Rayner when he was part of the Justice League while Hal Jordan was gone after Emerald Twilight. It just goes on and on.
Cyborg Superman, Hank Henshaw is essential in Hal Jordan's fall from grace, which was still canon in New 52 GL works. But in stories taking place in Comic Book/Supergirl, Zor-El (Supergirl's father) is Cyborg Superman and completely new to the world. Okay, let's just assume he's the second Cyborg Superman and Kara just doesn't know about the original because she's new to Earth... And then Superman: Lois and Clark happened and features the pre-Flashpoint Superman actively trying to prevent the New 52 Hank Henshaw from becoming Cyborg Superman in the present day when he returns from a space mission, so it's impossible that any Cyborg Superman existed when Hal turned evil. With the advent of Rebirth and especially Superman Reborn, Lois & Clark has been reworked and Henshaw is established as having been Cyborg Superman in the past and somehow was transformed back into his human form. He returns to his Cyborg Superman form using a gem... one he acquired in Lois & Clark when he was an ordinary human!
Legion of Super-Heroes: The Legion has went through a lot since the Crisis messed them up good:
Following the events of Crisis on Infinite Earths, there was a problem - their stories continued as is, but their past still considered Superboy an integral part of their history despite the fact that The Man of Steel retconned that away. This was initially solved by having the Time Trapper create an alternate universe pocket dimension where a Superboy lived. There would be a crossover with Superman that would end with Superboy performing a Heroic Sacrifice, setting the stage for The Supergirl Saga. The Legion would last that way until launch of the fourth Legion title.
In issue #5 of Legion of Super-Heroes Vol. 4, the Time Trapper is killed and, since his meddling created the Legion, the pre-Crisis Legion is now erased. In its place was now part of the "Glorithverse", where Mon-El, under the heroic name "Valor", became the inspiration for the Legion via heroics performed in the 20th century. This would last until Zero Hour: Crisis in Time!, when the Legion sacrifice themselves to give their future another chance despite the Entropy. This would effectively erase the last vestiges of the Pre-Crisis universe.
Following Zero Hour was the Reboot Legion, which returned the heroes to their old teenage selves and having new adventures despite using the books the old Legion used. This Legion would participate in Final Night and recruited the Conner Kent Superboy into their ranks. Ultimately, sales would drop and the decision would be made to reboot the Legion again.
After a storyline involving the Reboot Legion and the Teen Titans, a new Legion was created, more of a rebellious group who fought to get rid of the peace in their future before becoming normal heroes, being referred to by fans as the "Threeboot" Legion. The title would get a change up following Infinite Crisis with Supergirl joining their ranks... and then things start getting complicated.
Around this time, another version of the Legion started to pop up. While still adults, this Legion claimed to be the group active when Superman started heroics as a teenager (as Infinite Crisis restored his past as Superboy). This "Retroboot" Legion would hang around and play a role in the Superman titles before the Legion of 3 Worlds arrived, using the Reboot, Threeboot and Retroboot Legions. It was revealed that the Reboot Legion came from a world destroyed during the Infinite Crisis, the Threeboot Legion hailed from Superboy-Prime's Earth-Prime and the Retroboot was New Earth's and it would stay that way until the New 52 dropped.
Then New 52 hit, and after limping along for a while with poor sales, the final issue revealed the current Legion (the same Retroboot Legion from before the New 52) was actually the Legion of a world similar to Earth 2.
The next Legion, the "Bendisboot", started off with a snarl thanks to being conceived of while the original Legion was being brought back. At the start of DC Rebirth, Saturn Girl appears in the present day and is looking for Superman. This is clearly meant to be either the original or Retroboot Saturn Girl. However, behind the scenes, the delays with Geoff Johns bringing the Legion back eventually resulted in an entirely new Legion being created by Brian Michael Bendis. As a result, when the Legion returns, Saturn Girl acts familiar with Clark Kent like the Preboot or Retroboot Saturn Girl would, and the Legion is explicitly only returned to continuity because Clark Kent became Superboy... except the Legion shown is the Bendisboot Legion, who weren't inspired by Clark, but by Jon Kent suggesting the idea of a United Planets, and who barely met Clark.
Post-Crisis Power Girl went through sooo much of this - she's a Kryptonian, she's an Atlantean - she's a weird metahuman - nobody knows. Finally, it was declared that she was a survivor of the pre-Crisis multiverse and her Continuity Snarl was the universe trying to "fit her in" and failing. After Power Trip (2005) she is considered to be the Supergirl analog of the original (Pre-Crisis) Earth-Two. Yes, this means there's a second Power Girl in a duplicate Earth-Two. And during the New 52, Power Girl was again the Supergirl from Earth-2, only it's a completely different Earth-2 from the previous versions. In the aftermath of the DC Rebirth relaunch, this Power Girl went back to being the original Superman's cousin, and the New 52 version has been limboed. As of Dawn of DC, Justice Society of America (2022) says she was a JSA member on Prime Earth in the seventies (which would make her Prime Earth's first Kryptonian hero), while her back-up strip in Action Comics and her solo title have her as feeling like an outsider on Prime Earth, and generally don't portray her as a more established hero than the ones in the S-shields. (At one point she was unfamiliar with idiomatic English, although that was dropped quite quickly).
Supergirl's continuity is pretty tricky due to DC's decision to kill the original Supergirl (Superman's cousin Kara Zor-El) back in 1986, and then trying to replace her with different versions before returning to the original character concept. There have been several different characters to use the name, including Superman's cousin, an artificial lifeform created by a parallel dimension Lex Luthor from a pocket dimension, said lifeform merged with a human who is simultaneously an angel, Superman's possible future daughter and other versions of Superman's cousin. And some of those might be the same people, and some of them might never have existed. It does not help that DC has treated Pre-Crisis and Post-Flashpoint Supergirl as the same character in the aftermath of the Death Metal event. However, it's not really complicated if you only consider the Superman's cousin version.
The issues with Supergirl and Power Girl mostly stem from the same source - post-Crisis on Infinite Earths, there was a huge editorial push to keep Superman truly the last Kryptonian (evil ones notwithstanding), so anyone who wanted to revive Supergirl or Power Girl had to jump through a bunch of hoops to establish they're not "really" Kryptonians. Fortunately, this was relaxed by the mid-2000s.
Superboy: John Byrne's reboot of Superman caused all kinds of problems since, for one, his Superman was never Superboy. No Superboy to hang out in the 30th Century with the Legion of Super Heroes. Then you have all of the splintering of Superboy - clone, Superboy Prime, Pocket Universe Superboy, etc.
Green Arrow has had numerous minor, but confusing, problems since Oliver Queen came back from the dead.
The problems began when novelist Brad Meltzer wrote a Green Arrow story called The Archer's Quest centering upon Oliver Queen going on a road trip with former sidekick Roy Harper to retrieve items that could be used to discover his secret identity. The problem with that is that Oliver Queen hadn't had a secret identity in years! In fact, in the Quiver storyline written by Kevin Smith (which came out less than a year before Meltzer's story) the main piece of evidence Batman used to convince a resurrected and amnesiac Oliver Queen that he HAD been dead was newspaper articles which used his real name while discussing his death.
Another problem was the revelation that the whole Archer's Quest was a ruse and that Ollie had really been trying to recover a photograph which proved that he had been present on the day his illegitimate son Connor Hawke was born and that Ollie, ipso facto, was a dead beat dad. The problem is that this scenario is completely implausible given the circumstances under which Ollie originally found out that Connor (who he had been traveling with for a while before his death) was his son - he had been told by the truth by his best buddy Hal Jordan, who was (at the time) nigh-omnipotent with the power of all The Guardians Of The Universe Minus One. For Meltzer's scenario to make sense, we have to believe that Hal Jordan is capable of being able to see the DNA of a person by looking at them but is unable to tell when his best friend is lying about having no idea he had an illegitimate son.
Winick wrote a flashback scene where Connor's mom approached Ollie and was sarcastically wished good luck in trying to prove the baby was his in court. This scene apparently took place BEFORE the shipwrecking incident which inspired Ollie to become Green Arrow, as he tracks her down once he gets back to civilization and is there to have his photo taken with Connor before he has a fight with Sandra and walks out of her life again. What makes this truly awful is this scene was meant to bookend the Green Arrow: Year One mini-series by Andy Diggle. Suffice it to say that Green Arrow fans who have read that book find it hard to believe that the man Oliver is at the end of the story would ever abandon a child in need, much less his own son.
Judd Winick did a major disservice to the character when he decided to join Green Arrow and long-time girlfriend Black Canary together again off-camera, only to break them up. He did this by having Green Arrow suddenly decide to have a one-night-stand with the niece of a friend, despite the fact that Ollie was ready to propose to Black Canary not a few months earlier in the final chapter of The Archer's Quest by Brad Meltzer. Indeed, the dialogue in the scene where Ollie nearly proposes suggested that he and Dinah had gone out a few times since his resurrection but that she wasn't ready to date exclusively, let alone get married.
Judd Winick caused problems with his Heading Into The Light storyline, which was meant to be a lead-in to Infinite Crisis. In the end, there were so many issues with the storyline that DC Comics had to retroactively declare that Heading Into The Light took place AFTER Infinite Crisis, even though the story ends with a wounded Oliver Queen having visions of himself in other realities.
Some problems also sprang up over the issue in which Doctor Light drained Kimiyo Hoshi of her powers. She appeared in Infinite Crisis and Birds of Prey with her powers intact, while other stories ran with the premise of having her powerless. This was eventually handwaved in an issue of Justice League, which had Kimiyo mention that while she still retained her abilities, they were now malfunctioning and only worked on certain random occasions.
The Bat-Family has been rendered a near-complete mess thanks to the "soft reboot" of the New 52, where the Broad Strokes of previous continuity have been kept in numerous cases until explicitly contradicted. Among the changes are a compressed timeline, where superheroes have only been widely known to the public for five years. The problem is, Batman and company get up to a lot, and the franchise's reliance on sidekicks makes the passage of time a bigger factor than with other heroes, so cramming all of Batman's history into five years seems outrageous at the best of times. A Hand Wave is attempted by having Batman operate in secret an extra year before the rest of the hero community, but this only helps so much. Another Hand Wave eventually got given that the Batman timeline spanned over ten years, which once again didn't clarify matters since the original timeline had been fifteen. It is a complete mess:
Batman's eleven year-old son Damian has been retained. Damian was originally conceived after Bruce had become Batman, and it was reasonable for Bruce to have been Batman for at least eleven years in the previous continuity, but not in the New 52. Bruce is said to have operated for about five years at the start of the New 52, and by then Damian was already well into his tenure as Robin. The situation was left vague (Batman and Robin showed only four of his birthdays without specifying his actual age) until it was eventually implied that Damian was conceived after Batman's birth and that his mother Talia aged him artificially using technology from Apokolips. However, DC Rebirth later showed Damian celebrating his thirteenth birthday, which messed things up a bit. It would all prove moot after Superman Reborn expanded the timeline of the New 52 —and thus how many years Batman was active — which makes his age plausible.
Metamorpho mentions in an issue of Batman, Inc. that he used to be a member of the JLA. This was a Continuity Nod to Morrison's JLA (1997) run, which no longer exists in the current canon. It's also stated in a later issue of Justice League that the team's membership did not change at all during the 5 year Time Skip aside from a brief inclusion of Martian Manhunter, meaning that there's no way Metamorpho was ever part of the group.
In the previous continuity, Batman is "killed" by Darkseid using a special ability called the Omega Sanction, which actually unsticks Batman in time. During the year where Bruce slowly struggles back to the present, Dick Grayson becomes Batman and has Tim Drake move on to become Red Robin while Damian becomes the new Robin. The New 52 retains Bruce's disappearance and Dick and Damian's tenure as Batman and Robin while saying that Tim never took up the Robin mantle and immediately became Red Robin out of respect for Jason Todd's death, but leaves out the details on that time period until Batman and Robin eventually managed to simultaneously claim that: 1. Darkseid himself has not attacked Earth since the first arc of Justice League and 2. Batman was still killed by the Omega Sanction.
At the start of the New 52, it was established that Tim Drake had been Robin and was now called Red Robin. Then Teen Titans eventually contradicted this by saying that he was never Robin, and that he started as Red Robin. Then Rebirth happens, which is already weird with continuity, but had Bruce specifically state that Tim was never Robin... then Tim recounts his origin in a later arc, it's his pre-New 52 origin, complete with him having been Robin!
At the start of the New 52, Michael Lane is the current person going by Azrael, after Jean-Paul Valley, who was the second Batman. However, years down the line, Valley was introduced, and had only recently taken up the Azrael moniker — and Valley is established as having had no interaction with the Bat-Family up until that point, meaning he could never have been Batman! Sure, fine, whatever... except a later arc, "I am Bane" has Bane invoke the story where he broke Batman's back and Valley became the new Batman, "Knightfall"!
Dark Victory caused one as Arnold Flass from Batman: Year One is among the Hangman's victims. However, while that story is set in the past (with Gordon still being married to his first wife, Barbara), the earlier-published Batman: Legends of the Dark Knight Annual #2 shows Flass alive and well when Jim Gordon and Sarah Essen get married.
All of which seem to stem from Morrison's apparent uncertainty of whether or not their run has connections to the wider DC Universe. In Blackest Night, Bruce Wayne's skull is dug up and transformed into a Black Lantern to invoke an emotional response in all of Batman's allies while Dick Grayson is busy attempting to ward off all the undead villains that are attacking Gotham. Meanwhile, in Morrison's run, the Blackest Night is apparently not happening in Gotham or in London as Grayson had transported Wayne's body (which is kept under Wayne Tower as opposed to the unmarked grave all other DC works claimed it was buried in) to the latter in an attempt to resurrect Batman using a Lazarus Pit. Also, Morrison went on to reveal that the body of Batman wasn't his real body (it was made fairly evident even at the time that he wasn't dead for real), raising the question of how the League all formed emotional connections to a mindless, insane clone.
Batman #700, "Time and the Batman" features Chief O'Hara alongside Commissioner Gordon when they come to the aid of Batman, Robin, and Professor Nichols. The problem? Post-Crisis, the death of Chief O'Hara kicked off the events of Dark Victory, his death being a full year before Dick became Robin.
The series takes place during DV, caused another. While Infinite Crisis would render it moot by restoring Wonder Woman's status as a founding member of the Justice League, at the time, the status quo was that she's only become active around the time of Legends. The problem? Even with the idea of Hippolyta going back in time to fight in World War II during her tenure, Cheetah appears in the story — and is both clearly the Barbara Minerva werecat incarnation and explicitly stated to have fought Wonder Woman before in the story.
Author Appeal nearly caused a second one as Tim Sale wanted to draw the costume the late Darwyn Cooke had then recently gave Selina — despite the story taking place years before she was supposed to get it. DC and Sale compromised by giving Selina a similar, but not the same, costume after she temporarily lost her luggage for a couple of issues.
Batman: Legacy gets one due to canonicity change ups. The storyline treats this as the first time the Tim Drake Robin confronted Ra's al Ghul. However, Tim had met Ra's previously in the one-shot Bride of the Demon. The reason for the change was that, after the events of Zero Hour: Crisis in Time!, editorial decided to make the Birth of the Demon trilogy — Son, Bride, Birth — non-canon due to not wanting Batman to have a kid out of wedlock, which stuck until Batman (Grant Morrison).
There's always issues trying to work out how various appearances of characters with multiple books fit together, but this is particularly true for the Bat-Books in 2023. In Batman (Chip Zdarsky), when Bats returns to regular Gotham after the events of "The Bat-Man of Gotham" there's reference to the Orgham Building, so it's set after at least some of Ram V's run on Detective Comics. But in V's "Batman, Outlaw", Catwoman refers to her and Batman being "at war" lately, which is almost certainly a reference to "The Gotham War" in Zdarsky's book. And trying to interweave the two stories leads to the conclusion that Batman is simultaneously possessed by a demon which is being held back by his own force of will and the bat-demon Barbatos and slowly having his mind taken over by his emergency back-up personality, the Batman of Zur-En-Arrh. The inside of Bruce's head must be quite a place!
Aquaman: Following the post-Crisis reboot of the mid-80s, Aquaman's origin story was changed - where he was originally the son of lighthouse operator Thomas Curry and Atlantean queen Atlanna, his father was now the spirit of the Atlantean wizard Atlan, his name was changed from Orin to Arthur, and he was cast out to die near the surface during a tide for having blonde hair, a sign of the curse of Kordax. He had been reverted to his Silver Age origins circa Final Crisis, by which time he had been killed. Whether this was by continuity changes meant to be addressed or editorial overhaul is unknown, but it did work in favor of Arthur and the discrimination he faced for being half-human.
In the first issue of the New 52Teen Titans, it was stated that Tim Drake kept his history as Robin and that previous iterations of the team existed, with references also being made to past Titans teams in Red Hood and the Outlaws. Come the zero issue of Teen Titans a year later, and Tim's been retconned to have always been Red Robin and this is the first team of Teen Titans, with the collected edition of the first Titans arc outright removing the details that were retconned out. And as for the previously mentioned members of the Titans in Red Hood, so far the word is, more or less, that Dick Grayson, Starfire, and Arsenal (and possibly some others) hung around with each other, but never called themselves any team name.
The references to Gar (Beast Boy) and Garth (Tempest) made by Arsenal in Red Hood can now be considered retconned out, as Garth was later introduced as an infant in the Aquaman title and Gar is a hero just starting out in New 52 continuity (and also red-skinned and red-haired, as opposed to his classic green look). An early appearance of Gar (with green skin and hair) was also edited out in the TPB run of Teen Titans, along with a cameo of Miss Martian (who was recolored to be a white blonde girl).
A Teen Titans arc also featured the 70's Titan Lilith rebooted as a villain, with some of the team members seeming to know who she was. This would have been even more of a continuity snarl had Scott Lobdell been allowed to go through with his original plan: The villain was going to be Raven, who was responsible for founding the '80s team of Titans- who now no longer existed.
Speaking of Garth, he was actually later reintroduced in the pages of Aquaman as an adult, with no explanation given for the "infant" comment. However, he isn't a superhero, and is an Atlantean soldier who goes by the name Tempest.
The removal of the Titans' history also led to a comment in Batwoman being altered, where Bette Kane mentions that she'd been a member of the team and had fought Deathstrokenote although this itself had to be a retcon, as Flamebird never got such a chance or was on the main team. Of course, she could have also been bragging. The Titans reference was completely erased in the TPB edition.
All of these snarls from the New 52 reboot lead to the creation of Titans Hunt (2015), which is designed to not only give readers the "lost" Titans team mentioned, but also rerail personality traits lost with Donna Troy, Garth and Arsenal.
In the Day of Vengeance mini-series produced around Infinite Crisis, Bill Willingham had The Spectre murder the Lords of Order and Chaos, T'charr and Terataya. This caused Hawk and Dove to instantly become depowered. All other writers proceeded to ignore this for later stories, although it created confusion among fans as to how Hawk and Dove could still have their powers. T'charr and Terataya's deaths were a bit of a snarl in the first place, as both had previously died in the '90s Hawk and Dove series— and had their powers sealed into the two heroes. Holly Granger's existence is another snarl in H&D history, as Dawn Granger was always said to be an only child in the '90s series. Tellingly, after Holly died and the New 52 reboot happened, it would appear that Dawn is back to having no siblings.
Countdown to Final Crisis is absolutely full of this. It laid claim to being "the spine of the DC Universe," meaning that it referenced many other events going on at the time. This meant that if two events were running simultaneously, Countdown would claim they occurred simultaneously... even if that was impossible. It really doesn't help that, despite having been heavily interwoven with the DCU of its year, Final Crisis, the event it led into, mostly ignored it; for instance, it's very hard to jibe anything that happens in Death of the New Gods with the depictions of the New Gods in Final Crisis. One fan, when attempting to try to order the DCU's stories chronologically, summed it up pretty well.
For example: if you read The Lightning Saga in JLA #8-10/JSA #5-6, it appears to take place as an unbroken sequence of events. Likewise, if you read Amazons Attack (and if so you deserve great sympathy), it appears to take place as an unbroken sequence as well, dominating everyone’s attention for several days. But if you read Countdown, and assume (not unreasonably) that it relates events in at least roughly sequential order, then based on when it chooses to have scenes tying in to these other storylines one would have to conclude that the JLA and JSA teamed up and set out to find the Legionnaires... then put that on hold when Diana got kidnapped and the Amazons attacked DC... then (somehow) put that on hold to get together again for the climax of "Lightning Saga" and Wally West's return (which All-Flash #1 tells us corresponds with Bart Allen’s death), and then all headed off to Keystone City for Bart’s funeral... before returning to DC and turning their attention to the Amazons again... and then left that alone again (with no sign of any resolution) and turned to other business like, e.g., going to work at the Daily Planet or spying on Mary Marvel... all before the final scene of "Lightning Saga" in which the Legionnaires leave our era. Oh, and all in the space of about four days. Make sense to you?
When Bart Allen became the fourth Flash, it's shown that Barry Allen, the second Flash and Bart's grandpa, had an active role in Bart's upbringing in Bart's series The Flash: Fastest Man Alive. This flies in direct contradiction with what has been previously said: Barry died before seeing his kids reach adulthood, never mind Bart. This might be explained with Infinite Crisis happening and retconning some of Bart's upbringing... but when Barry returns in The Flash: Rebirth, Bart is outright hostile towards him and the subsequent Flash series has him try to be closer to Barry because they were never close. Given the unpopularity of The Flash: Fastest Man Alive, it's likely that Geoff Johns (the writer of The Flash: Rebirth and the subsequent Flash series) just ignored its retcon, but it's still jarring if you read them back-to-back.
The West family in the New 52. Initially, Wally West was the thirteen year old nephew of Iris West, son of her brother Rudy West, as he had been pre-Flashpoint. He had a Cool Uncle in the form of Daniel West (the Reverse-Flash). However, DC Rebirth featured the return of the original Wally West, who is established to be the son of Rudy and Mary West as he'd always been, and both Wallys were named after their great grandpa, Wallace. Wally West II (the New 52 Wally) is retconned into being the son of Daniel... except Daniel very clearly established that he's, at oldest, about 24 when then-current stories take place, as he was caught committing a robbery by the Flash on his eighteenth birthday. Meaning he had a son at the age of eleven! This was eventually softly retconned by giving Daniel an Age Lift, as an adult Daniel was present alongside Rudy and Iris for Wally West's birth.
In The Flash: Rebirth, Barry Allen learns about two other forces that act like the Speed Force; the Sage Force and the Strength Force. At the same time, in Justice League (2018), Lex's new Legion of Doom is harnessing seven hidden forces that are the opposite of the League's powers, including the Still Force, which is the reverse of the Speed Force. A few issues into the Flash story, Barry says he's also recently learned there's a Still Force which is probably related to the Speed, Stage and Strength Forces as well, but a) it's not clear how the Justice League story could have happened during the Flash story, b) the idea that there are four related forces doesn't tie in with the LOD's equal and opposite theme and c) there's no mention in Flash of Grodd harnessing the Still Force, which instead empowers a new character called Steadfast. They do their best, but there's a definite sense of trying to tie together two concepts that were just similar enough to contradict each other if they didn't at least make the attempt.
Between the events of DC One Million and the "World War III" arc, the Martian Manhunter officially took a sabbatical from the League because of the events of the former. Mark Millar, Mark Waid, and J.M. DeMatteis were all too wiling to ignore this when they guest wrote, having J'onn still be present (and this does include the issue Millar guest wrote that had J'onn and Batmanknow aboutSailor Moon)note and is in fact, the biggest offender as the third Hourman came back in time, who was meant to sub in for J'onn, told them about the events of the upcoming "Crisis Times Five" and J'onn and Bruce were meeting with Superman to discuss this very thing.
It was subjected to this again as Morrison's run ending with the Big Seven heading off to face Dr. Destiny, both an And the Adventure Continues ending and as a Bookend to the fact that Justice League: A Midsummer's Nightmare helped set-up Morrison's run and featured (an albeit unwilling) Dr. Destiny being confronted by the Big Seven. Waid's run, which included issue 50note the issue where Batman rejoins the League after being kicked off in the aftermath of JLA: Tower of Babel, depicted the League's fight with Dr. Destiny in that issue as the first time they've met since A Midsummer's Nightmare.
Morrison's run itself has the Corinthian's skull seen in Daniel's chest — despite Morpheus already restoring the creature during the events that led to Daniel replacing Morpheus as Dream in the first place.
The New-52 and Rebirth Harley Quinn runs didn't even attempt to explain how Harley could be having random adventures in Coney Island while supposedly also being a Boxed Crook in Suicide Squad, although there were a couple of audience-teasing references to the issue. The relaunch of the series in 2021, after the "Joker War" Batman event, explicitly stated that Harley had been in the Suicide Squad, but had now been released.
Is Wondy the child of Hippolyta who created a statue with Athena's guidance which was animated by a miracle from Aphrodite? Was she animated by all the goddesses? Or is she Hippolyta's natural-born child by Heracles, who seduced the Amazon Queen and stole her girdle? Or was it rape? Is she actually Zeus's kid?
Who was Wonder Woman in World War II? Was it Diana? Was it Hippolyta who took the Wonder Woman sobriquet and joined the JSA, or did she stay on Paradise Island and never set foot in man's world? Was there no Wonder Woman in World War II at all and it was actually Fury I replacing her? Or did both Hippolyta and the Fury work together?
Where did the Amazons come from? Did Aphrodite make them on her own? Are the Amazons Mars and Aphrodite's children? Did Mars make all the men and Aphrodite all the women in the world? Do they adopt women from outside the island or are they deeply isolationist? Are the other Amazons superpowered or is their physical prowess merely peak-human? Are they immortal? Are they only immortal as long as they don't fall in love? Are they only immortal if they stay on the island? Do they reproduce by raping sailors? Do they not reproduce at all? Is their culture based around combat or love or peace or what?
Is it Paradise Island or Themyscira? Is it real at all? If it's real, did Steve Trevor crash on it during World War II and convince the Amazons they couldn't ignore the outside world or was that his mom?
And then there's actual mythological characters, who not only get represented out of character with their original myths but also don't even get consistent characterization between writers.
, when attempting to try to order the DCU's stories chronologically, summed it up pretty well.For example: if you read The Lightning Saga in JLA #8-10/JSA #5-6, it appears to take place as an unbroken sequence of events. Likewise, if you read Amazons Attack (and if so you deserve great sympathy), it appears to take place as an unbroken sequence as well, dominating everyone’s attention for several days. But if you read Countdown, and assume (not unreasonably) that it relates events in at least roughly sequential order, then based on when it chooses to have scenes tying in to these other storylines one would have to conclude that the JLA and JSA teamed up and set out to find the Legionnaires... then put that on hold when Diana got kidnapped and the Amazons attacked DC... then (somehow) put that on hold to get together again for the climax of "Lightning Saga" and Wally West's return (which All-Flash #1 tells us corresponds with Bart Allen’s death), and then all headed off to Keystone City for Bart’s funeral... before returning to DC and turning their attention to the Amazons again... and then left that alone again (with no sign of any resolution) and turned to other business like, e.g., going to work at the Daily Planet or spying on Mary Marvel... all before the final scene of "Lightning Saga" in which the Legionnaires leave our era. Oh, and all in the space of about four days. Make sense to you?