The interactions between Joker and Penguin inside Penguin's night club, The Iceberg Lounge, in "Joker's Millions" become this in one of the "Arkham Stories" during Batman: Arkham City, where they were on much harsher terms (and Penguin murdered and stuffed Joker's hyenas).
Just try watching "Over the Edge" after Batman: Arkham Knight and season 3 of Arrow, and knowing what was All Just a Dream in the former is mostly reality in the latter two, as all three involve similar plot points: the daughter of a cop ends up having something bad happening to her (DCAU!Barbara/Sara Lance getting killed/Arkham!Barbara Gordon ends up getting kidnapped and seeming commits suicide after a dose of fear toxin), her father (DCAU and Arkham!Commissioner Gordon/Quentin Lance) ends up getting pissed at someone (DCAU and Arkham!Batman/Ollie and Quentin's other daughter Laurel) for keeping a secret (Barbara's double life as Batgirl and Oracle/Sara's death itself) from him and decides to cut off ties from the hero, and the hero's identity gets outed (DCAU!Gordon learning the truth from Barbara's computer/Arkham!Scarecrow forcing Gordon to remove Batman's mask on live TV/Arrow!Ra's al Ghul decided to frame the Arrow and tell Quentin Ollie's the Arrow). In "Over the Edge", the whole thing turned out to be a nightmare that Barbara had after getting dosed with Scarecrow's fear toxin; in Knight, Bruce was tripping on Scarecrow's fear toxin and hallucinated a similarly-dosed Barbara committing suicide, only to learn the real Barbara was still alive, but the rest really happened; and in Arrow, Ollie is saved when Roy, in part due to guilt for killing a cop due to a Mirakuru-fueled rage, decided to take the fall for Ollie and Sara would later be resurrected. But perhaps the worse thing isn't from an Alternate Continuity, but from Sequel SeriesBatman Beyond, as the episode "Eyewitness" sees Barbara herself getting tricked into thinking Terry killed Mad Stan by Spellbinder and go to the very lengths she feared her father would go before Terry cleared his name.
In "Make 'Em Laugh" the Joker uses microchips to alter the minds of innocents to make them criminals. ThencomesRotJ, where he uses said technology again to terrible effect, this time on Tim Drake himself.
In the episode "Be A Clown", Mayor Hill's son Jordan is kidnapped by a disguised Joker and tricked into performing criminal acts against Batman. Jordan is horrified when he finds out. In Return of the Joker, the Tim Drake Robin is manipulated by the Joker via Mind Rape into becoming Joker Jr., later taking on the role after he kills the Joker either directly (original) or in a freak accident (recut); and causes chaos in Gotham in the years that followed.
The ending of "Harley's Holiday," in which Batman and Harley Quinn share a tender moment with each other sympathizing over each other's respective bad days becomes this as despite this moment, Harley still ends up helping out Joker with his plan to kidnap and brainwash Tim.
Another episode worsened by Batman Beyond: Return of the Joker is "I Am the Night", where Batman angsts over the possibility of his friends and allies being hurt... including Robin (at the time of the episode, Robin is Dick and not Tim, but still).
Yet another episode worsened by Batman Beyond: Return of the Joker is "Joker's Favor". Charlie Collin's Heroic Heelization Speech is horribly right because Batman will not be the one who brings out Joker's demise. Instead, Joker is killed by a Jokerized Tim Drake, who Joker had intended to shoot Batman.
Charlie Collins: You're not getting caught. Not this time. I found this blown out of the van. (reveals a Joker bomb) This is how it ends, Joker. No big schemes. No grand fight to the finish with the Dark Knight. (...) Kinda funny. Ironic, really.
A subtle instance comes from the Batman Beyond episode "Out of the Past", in which Terry takes Bruce to see a musical about his tenure as Batman. Said musical opens with a musical number of Batman confronting several villains who have kidnapped Robin and taken him to Arkham, among whom are Joker and Harley.
Batman's origin story in this continuity (as well as others), as of July 20, 2012. In this continuity, Batman's parents were murdered by a thug (implied to be Joe Chill, but he was never named) shortly after seeing a film. The film he saw? Gray Ghost, which was the in-universe inspiration for that continuity's Batman. Flash forward to July 20, 2012, several people were killed by a madman in a shooting at a Century theater in Aurora, Colorado. The movie that was being shown on that date? A midnight showing of The Dark Knight Rises.
In "Paging The Crime Doctor", Rupert Thorne needs heart surgery. His actor, John Vernon, died in 2005 of complications from heart surgery.
In "The Laughing Fish", the Joker tries to have his Joker Fish copyrighted, and the patent guy tells him he can't copyright food. Twenty years later, Monsanto wants to do just that.
Meta example from "I Am The Night" - Batman laments the fact he's merchandised to death, on t-shirts and action figures, despite his intent. Not only was the cartoon used to sell a toy line, but not long after Warner began a massive ad campaign to push Batman toys, shirts, and other merchandise onto children.
In "The Man Who Killed Batman", the Joker doesn't believe that Batman seemingly died for good and tries to prove the rumors wrong by staging a jewelry store robbery. When the Caped Crusader fails to show, Joker becomes so depressed that he orders his henchmen to put the jewels back in their proper place, and then holds a funeral with a genuine heartfelt eulogy of how Batman completes him. After the sudden passing of Kevin Conroy in November 2022, Mark Hamill mourned the loss of his close friend and later announced that he won't play the Joker anymore as he no longer has his Batman to play against.
Joker: "Without Batman, crime has no punchline."
Mark Hamill: "They would call and say, 'They want you to do the Joker,' and my only question was, 'Is Kevin Batman?' If they said yes, I would say, 'I'm in.' We were like partners. We were like Laurel and Hardy. Without Kevin there, there doesn't seem to be a Batman for me."
The film showing how Bruce is seemingly fated to be alone looks even worse when Batman Beyond shows this really will be the case for him.
The Justice League Unlimited episode "Epilogue" showed that Andrea didn't give up being the Phantasm, becoming a hitwoman.
The scene where Bruce visits his parents' grave is an extra twist of the knife when you think of the DC Rebirth storyline The Button, in which his father (from another timeline) tells him personally to stop being Batman and find happiness. The end of Tom King's run on Batman (Tom King) makes it worse as The Thomas of Flashpoint is the final villain in a Big Bad Duumvirate with Bane and tries to force Bruce to quit, resulting in Alfred's death by Bane's hands. Additionally, in the Batman tie-in for Flashpoint, Martha (who became that reality's Joker) was Driven to Suicide when Thomas informed her it was supposed to be Bruce who became Batman.
The second episode has a passenger plane hit with a missile and nearly crash into Metropolis. To make it worse the missile was fired by a terroristnote okay so they're more like mercenaries, but everyone called them terrorists in the episode. Similarly, the first part of "The World's Finest" has a scene where terrorists take over Air Force One.
The finale of "World's Finest" has the Joker firing missiles at multiple buildings, sending them crashing to the ground.
Dan Turpin being killed by Darkseid can be this if you read Final Crisis, in which a dying Darkseid pulls a Grand Theft Me on Turpin himself.
The shootout in the nightclub is this, given the Orlando nightclub shooting.
What Tim goes through here in the flashback, given that the backstory for Batman: Arkham Knight has Jason Todd go through the same thing and end up assisting the Scarecrow in, what is for all intents and purposes, a terrorist attack on Gotham.
In one episode, Adam Evans insists on using fireworks for his concert despite warnings from his crew about using them on a small stage. When the fireworks go off, they end up setting the place on fire. This became a whole lot harsher when the Rhode Island nightclub "The Station" got set on fire in February 2003 during a Great White concert, and killed at least 100 people, due to a combination of polyurethane insulation and the band's use of pyrotechnics on a smaller than normal stage.
Hawkgirl hitting John Stewart's head in "A League of Their Own". Especially when you know what happens in "Starcrossed".
When Wally is almost taken by the Speed Force but is saved by his friends. It didn't turn out as well for his Young Justice counterpart.
In Part 1 of "The Savage Time", an alternate-universe Batman leads a resistance to fight a regime, and is helped by the other members of the League. Same thing happens in Injustice: Gods Among Us, except some of those League members are leading the regime.
In Part 2 of "Hearts and Minds", John Stewart complains to Hawkgirl that Katma, his old instructor, still thinks of him as a raw recruit. Hawkgirl calmly notes that "we tend to cling to images from when we knew them best, forgetting that they do change". This comes off a bit harder to take after Shayera's own ordeal with this in "Starcrossed",
The League calling out Batman on his I Work Alone attitude becomes a bit harsh when in Justice League: Doom he goes through with quitting the team.
Robert Foxworth voicing a character who bares Fantastic Racism against the Justice League and part of an opposing secret government agency would eventually become ironic as Foxworth would later voice Ratchet in the Transformers Film Series and unfortunately would end up on the receiving end of the wrath of a Fantastic Racist government agency who would eventually kill him around the beginning of Transformers: Age of Extinction.
In the episode "In Blackest Night", when The Flash offers to speak for John Stewart, who is being accused for the destruction of a planet, the alien judge reminds him that if the accused is found guilty, anyone representing the accused will share the same penalty. Sounds exactly like the Defense Culpability Act, doesn't it?
"Kids' Stuff" was a largely lighthearted episode dealing with a child making the world a sort of Neverland, but any negative side effects of losing all the adults on the planet for an unknown period of time isn't really addressed for the sake of that tone. When Klarion the Witch Boy pulls a similar stunt in Young Justice we actually do see exactly how terrifying (and dangerous) it was for both sides of the adult/child split.
In "Savage Time", Diana crosses paths with Steve Trevor while having an escapade in World War II. The two are forced to part once Diana and the League complete their mission and return to the present day, but Diana searches him up and reunites with him at a retirement home. Come Wonder Woman (2017), where we already know that Diana lives to present day, and we see that Steve never lives to old age, Diana witnesses his death, and in the present day, all Diana has left of him is a photograph.
Steve: I wish we had more time. I love you.
In the episode "Far from Home", there's a subplot where it's feared Supergirl would die after traveling to the future. Sequel film Justice League vs. The Fatal Five, Star Boy travels back to the past and unlike Kara, who merely chose to stay in the future, Thomas really did die in the past, far from home.
Joss Whedon productions in voice acting roles on the DCAU shows such as Nathan Fillion and Amy Acker on Justice League becomes uncomfortable in hindsight as Whedon would take over directing from Zack Snyder on Justice League (2017) for reshoots, but was reported on Twitter for being an abusive and unprofessional Prima Donna Director by Cyborg's actor Ray Fisher.