This is a mainstay of Astro City, which gives the series a very strong sense of being grounded. By way of an illustrative example, virtually everything from the very first issue is called back in future issues:
Samaritan briefly mutters his flight time when he arrives to visit Steeljack in "The Tarnished Angel," and again when visiting Maddie in "On the Sidelines".
Samaritan mentions that Honor Guard's alien detector is on the fritz. This seeming throwaway line is a critical plot point in "Confession."
At the Honor Guard meeting Cleopatra mentions the rising threat of "Gnomes" in the mountains. A few issues later a giant, Gnome-built robot squares off with some heroes.
Samaritan's fight with the Living Nightmare is the front-page story for the Rocket in the very next story, "The Scoop."
And the Rocket's lead story in issue one is "Jack-in-the-Box captures Brass Monkey." Both Jack and the Monkey of course appear in future issues.
At the office, Asa's latest work assignment is about the First Family, who of course also make frequent appearances throughout the series.
A news story in "Confession" mentions yet another award ceremony in Samaritan's honor.
In the science-fiction "sequel" The Rebels, Gestrelle Luricahn owning elfin artifacts is just part of her job/field of research. The fact that nearly all of those that we see decorating her room had some importance in previous ElfQuest stories is a Continuity Nod.
Sometimes panels from earlier comics get "paraphrased", using the same composition and (if possible) characters; for example one in ''Kings of the Broken Wheel'' 3 is modeled after one in the very firstElfQuest story ever.
In a montage of the Wolfriders' dens in Wild Hunt, Tyleet's can be identified by the ornament that her long-dead adoptive son used to wear as a headband.
In The Beano Annual 2009 the Ratz (a group of rats from a 2000s Beano comic strip) briefly meet the Nibblers (a group of mice from a 1970s/1980s Beano comic strip).
In Mortadelo y Filemón, any appeareances of returning villians are punctuated by a side note pointing to the last story in which they starred. And then there is the book Venganza Cincuentona where a dozen of the most iconic Monsters of the Week return to fight the heroes together.
With the return of changelings, naturally the wedding episode gets brought up. Apparently Rainbow Dash barely remembers it because she had too much of Pinkie's Tutti-Frutti Sherbet Sugar Punch.
Much of issue #2 is spent in a mine that once was worked by the Diamond Dogs. Several of the background elements allude to their dog-nature (bones and doghouses).
In issue #2, when the Changelings work to create discord between the Mane Six, Pinkie is told that she is annoying to her friends, and temporarily reverts to her "Party of One" straight-hair look.
The Cloud Gremlins from the Rainbow Dash micro-comic appear in Rainbow's nightmare.
Something of a minor Call-Forward; the hat Rainbow Dash wears in Fluttershy's nightmare in #6 is the same hat she wears at Twilight's coronation.
Rainbow Dash considers what Daring Do would do in their imprisoned situation.
In My Little Pony Micro Series Issue #3 Twilight mentions that after the events of Lesson Zero, she of all ponies knows how too much work stress can make a pony snap.
When Rarity first leaves for her retreat, Pinkie tells her to not forget to write while glaring at Rainbow Dash, with Dash sheepishly stating that she meant to.
Luna appears on Rarity's dreams, singing "Hush now, quiet now".
The line "The pony everypony should know" (from "Sweet and Elite") is repeated a few times when Flax and Wheat are whisked off by the Elite.
The cover of the magazine AJ is reading at the start features the cleaned-up Hayseed Turnip Truck, from the 2-page short at the end of the comic.
The short story makes a direct reference to Hayseed's brief scene in Sweet and Elite.
When the Simpsons are traveling through space onboard the Planet Express ship, Marge expresses how amazed she is to see space like that. By contrast, Homer looks bored and says "Meh. You've been in space once, you've seen it all.", referencing the one time he was in space. This is followed by him reminding Marge that she's afraid of flying. She then starts freaking out by running back and forth and yelling "Let me out!" repeatedly, just like in "Fear of Flying".
At Farnsworth's laboratory, Bart takes a peek into the box that contains the Futurama universe from "The Farnsworth Parabox".
At the Planet Express building, Bart finds the finglonger, which was introduced in "Anthology of Interest I".
In the two-part story "Cloak and the Dagger", the Vulcans of Darien 224 use extremely powerful psionic weapons such as the Tol par-doj and the Vorl-tak. The Star Trek: The Next Generation two-parter "Gambit" established that the Stone of Gol was an ancient Vulcan psionic weapon of immense power.
In "Immortal Wounds", Toluk, the Vulcan ambassador to Neyda Prime, has heard of Spock and is aware that he chose to attend Starfleet Academy instead of the Vulcan Academy over his father Sarek's objections, as was established in the Star Trek: The Original Series episode "Journey to Babel". Ambassador Toluk later mentions the Vulcan concept of the katra.
In "One of a Kind", there is an internal nod. As he prepares to leave the Enterprise, Nano packs a photograph of himself at a party or celebration with Yeoman Dermot Cusack, who was killed in "Our Dearest Blood", as well as Moves-With-Burning-Grace, José Tyler and Gabrielle Carlotti.
In "The Fallen, Part One", Captain Pike visits his hometown of Mojave, an illusion of which was created for him by the Talosians in "The Cage".
Also in "The Fallen, Part One", the gargoyles on the Empire State Building remind Spock of Taguan effigies of the Great Bird of the Galaxy.
Spock's reckless actions over several arcs, including the "Return of the Archons" and "Operation: Annihilate!" storylines, are later brought up by Uhura as she attempts to console him over his lingering feelings about the destruction of Vulcan.
Quocch (a minor character from the Nero comic miniseries, and a deleted scene from the 2009 film) is seen explaining how he escaped from Rura Penthe at the very beginning of the "Vulcan's Vengeance" story.
"After Darkness" has several references to the video game adaptation, including Bones getting exasperated once again about landing on the planet surface and Kirk trying to reassure Spock and Uhura by saying that he hopes they don't run across any more Gorn. In issue #24 (a standalone story based off the TOS episode "Arena"), Kirk gives a personal log where he complains of having Past Experience Nightmares about his previous encounter with the Gorn.
To Boldly Go's first issue shows Jaylah and the cadets from the Starfleet Academy companion miniseries in class listening to Scotty's lecture.
In "Renewal", the commander of the Klingon battle cruiser who attempts to capture the Enterprise is Krell, who previously provided the Villagers, one of the factions on Neural, with flintlocks to fight the Hill People in "A Private Little War".
Also in "Renewal", both Kirk and Krell refer to the Organian Peace Treaty, signed as a result of the Organians' intervention in "Errand of Mercy" and first mentioned in "The Trouble with Tribbles". Krell relishes the idea of abandoning the treaty and engaging the Federation in open warfare but Kirk is concerned that the Organians seem to be no longer willing to enforce it.
In "Worlds Collide", Dr. McCoy reminds Kirk that the Enterprise saved Miramanee's planet Amerind from being destroyed by an asteroid in "The Paradise Syndrome" and asks him why they can't do the same thing in the current situation.
When Saavik comments that she is aware that a mind meld is a deeply personal experience in "Worlds Collide", Spock admits that he has never melded with his father Sarek, as was revealed in the Star Trek: The Next Generation episode "Unification, Part 2".
In "Silent Cries", Uhura, Sulu, Scotty and Chekov each give their pick for the most frightening mission that they've had aboard the Enterprise. Uhura picks her visit to the Mirror Universe in "Mirror, Mirror", Sula picks being stranded on Alfa 177 at 120 degrees below zero in "The Enemy Within", Scotty picks having to cold start the warp engines before Psi 2000 disintegrated and destroyed the ship in "The Naked Time" and Chekov picks Khan Noonien Singh taking over the Enterprise in "Space Seed". Sulu is surprised by this as he claims that Chekov was not aboard the Enterprise then. However, it turns out that Chekov was assigned to Engineering at the time, which even Scotty doesn't remember. This accounts for the fact that Khan recognizes Chekov in Star Trek II: The Wrath of Khan.
Sunny Series: Swing It, Sunny shows in Sunny's room the crochet doll that Teezy and Ethel gifted her in Sunny Side Up, as well as the bicentennial celebration flag.
is modeled after one in the very first