Chowder has one scene where Chowder is being interrogated by the police, and he breaks down and begins confessing to everything. This includes a reference to a previous episode where Chowder's brain becomes huge, realizes he's in a cartoon, and turns the show into a talk show.
The DCAU had tons of Continuity Nods throughout its run.
The entire Cadmus Arc from Justice League Unlimited is essentially based off of the last episode from Superman: The Animated Series, and indeed, its conclusion refers to events as far back as the middle of the show's second season.
A classic from Batman: The Animated Series; in the episode "Pretty Poison", Pamela Isley, (aka Poison Ivy), is dating Harvey Dent (the future Two-Face), intending to kill him for destroying a nature preserve to build Arkham Asylum. Later in "Almost Got 'im", Poison Ivy and Two-Face meet in a Bad Guy Bar and start trading insults. Ivy then turns to Joker, Penguin, and Killer Croc and explains, "We used to date." (There's a bit of dramatic irony here in that Joker, Penguin, and Croc all respond with a casual "Ah....", probably thinking something along the lines of "she must have been a total bitch to him." They don't even realize that she almost murdered him!)
On Batman Beyond, the Batcave is filled with relics from Bruce Wayne's crimefighting career, in the original Batman: The Animated Series. This merges with Chekhov's Gun when Bruce uses Mr. Freeze's cryogenic gun to subdue a villain when she invades the cave. In another episode he dons the goggles and hat of the Grey Ghost, his childhood hero, to mask his identity.
In the same show, there is an odd example of a Mythology Gag becoming a sort of retroactive Continuity Nod: the elderly Bruce Wayne's pet Great Dane is named Ace, which was also the name of the Bat-Hound, Batman's pet dog who appeared in his comics during the 1950s and 60s. In a later episode, though, we learn about one of his later adventures as Batman, and the name of "Ace" takes on a whole new meaning: Namely, the suggestion that Ace is named after Ace of the Royal Flush Gang, whom Batman stayed with as she slowly died in the Justice League Unlimited episode "Epilogue".
One of these relics actually makes a pretty good explanation for the unexpected reappearance of a certain character at the end of Return of the Joker. Even though said reappearance was a last minute decision. When Inque infiltrates the Batcave during the series, Harley Quinn's costume can be seen among the trophies. Even though not stated directly by the writer, it created the idea among fans that Barbara might have lied to Terry about Harley Quinn's end. That she either didn't fall into the chasm, Batgirl managed to pull her up, or that they actually checked for her body and found her alive and covered up her survival to prevent Tim from taking revenge on her. This could also be seen as unintentional continuity nod.
The Splicers, people who use LEGO Genetics as a form of body-art akin to tattoos & body piercing use technology descended from that used to create Man-Bat, as well as the various cat monsters in "Tyger, Tyger".
A very nice touch is the fact that the relic given pride of place in the cave (apart from the suit rack) isn't a trophy from a case, it's the costume of The Gray Ghost, Bruce Wayne's in-universe inspiration for his vigilante career.
The 200/201 two-parter is full of references to the events of past episodes, making it a perfect Milestone Celebration.
Bill Cosby (a robot from the future, It Makes Sense in Context) wants to be Cartman's friend in "Trapper Keeper". Cartman responds that he shouldn't have friends that are over 30 as he kind of got screwed on that one, a reference to "Cartman Joins NAMBLA".
"Free Willzyx" focuses on getting a killer whale to the Moon (as the boys are tricked into believing the Moon is their natural habitat). The whale is shown dead on the Moon's surface during the credits. Several series later in "201", Tom Cruise gets sent to the Moon where his corpse is displayed next to the whale's. The next series, both the whale and Tom Cruise's bodies are visible during the shot of the DP moonbase in "The Coon Part 2: Captain Hindsight".
Several episodes had nods to Kenny's deaths. Cartman's quote of "he dies all the time." in "Cartmanland", Stan saying "Who didn't see that coming?" after Kenny dies in "Fourth Grade", then saying "That was a good one." in "Chef Goes Nanners" and treating it as unimportant when he was killed in "Underpants Gnomes", Timmy's likely knowledge of it after giving Jimmy a Kenny-style parka in the hopes that it would kill him, Kenny's mother giving birth to a new Kenny and her and her husband bringing up how many times this had happened, and Kenny's own knowledge of it when he is insulted that Stan treats Kyle's impending death more seriously than he ever treated his (just before he gets killed by a falling piano). Though a recent Cerebus Retcon has placed most of these as Negative Continuity, or just coincidences, as it reveals that Kenny's deaths are a super power of his that cause him to die and come back over and over with no one but him remembering.
In "AWESOM-O" Ms. Cartman says Eric is supposed to be grounded for trying to exterminate the Jews two weeks ago. He did just that in "The Passion of the Jew", which aired two weeks earlier. Butters also mentions events from "Jared Has Aides" and "Casa Bonita" to illustrate how mean Cartman is.
Butters help Cartman compile a list of things to atone for in the Season 9 episode "The Death of Eric Cartman." Many events were seen in episodes from the previous season such as "Up the Down Steroid" and "Pre-School". Then there were references to "Scott Tenorman Must Die" and "Kenny Dies" from season 5, and "the Sally Struthers incident", which happened even earlier.
In "Terrence and Phillip: Behind the Blow", Stan mentions that he and his friends saved Terrence and Phillips lives once. There was also mention of the Canadian-American War of 1999.
"Do the Handicapped Go to Hell?" and "Probably" revisit Saddam's relationship was Saddam.
In "The Entity", Bill Gates has a hole in his head from when he was shot.
"The New Terrence and Phillip Movie Trailer" is about the boys trying to see the trailer for Asses of Fire 2.
In "1%" an Okama GameSphere can be seen in Token's room.
In the Futurama episode "The Sting", Fry's funeral includes many guests from previous episodes — including the fossilized Seymour, the women he had been with (including the radiator from "The Lesser of Two Evils"(It Makes Sense in Context)), and other characters and plot points. The DVD movies are just full of nods.
Futurama is basically one big nod. One of the best is one throwaway line in which Bender states that "bending" is his middle name — meaning, of course, that he's good at bending. He then says that his full name is "Bender Bending Rodriguez" due to being made in Mexico, which any other show would never mention again and/or directly contradict in other episodes. Futurama, however, makes the name canon and mentions it often, even having an orphanage named the "Bender B. Rodriguez Orphanarium".
Zoidberg:(trying on a Mariachi replacement shell) Hey gringos! Here comes El Zoido to poison your drinking water! Bender: Hey, I'm Mexican and I find that offensive!
In the episode "Lrrreconcilable Ndndifferences" Lrr asks Leela "Do I know you?" and she says "Yes I was in your mouth for about 5 minutes" referring to the episode "The Problem with Popplers" where she discovers the "Popplers" which are actually the Omacronian newborns.
In "Time Keeps on Slippin'", Fry tries to figure out what he did to win over Leela's heart (he can't remember due to time skips). Hermes suggests that maybe he's a fantastic lover; Amy quietly says "No.", referencing their brief relationship in "Put Your Head on My Shoulder".
In "The Late Philip J. Fry", the professor creates a time machine that can only move forward in time. His reasoning is so that "That way you can't accidentally change history or do something disgusting like sleep with your own grandmother." Fry, who did exactly that in "Roswell That Ends Well", seems to agree with this logic.
In many episodes there are small bits in the background which recall earlier episodes — the enormous stone head that was in their basement for years, the electric hammer showed up in one of the Treehouse of Horror shorts, "Homer's Enemy" had Frank Grimes in complete disbelief when he saw Homer's Grammy award and photos from his time with NASA, and so on. However, despite these things, The Simpsons usually has Negative Continuity. It plays them against each other, like the episode where Homer took off Ned Flanders' roof to act as a snowplow, and Flanders complained that Homer used to be a professional snowplower and still had the equipment (and theme song). Similarly, Mr. Burns and Krusty the Clown never recognize Homer and Bart, respectively, even when all the previous episodes they've interacted in are mentioned.
Smithers: That's Homer Simpson, sir. All the recent events in your life have revolved around him in some way. Mr. Burns: Simpson, eh?
Krusty: Yeah, well, what have you done for me lately?
Bart: I got you that Danish.
Krusty the Clown: And I'll never forget it.
Earlier episodes featuring Sideshow Bob as the antagonist would typically include a brief recap of his past evil deeds.
In the movie, towards the end when Homer and Bart jump Springfield Gorge on a motorcycle, you can see an ambulance crashed into a tree - the same one that carried Homer away to hospital (well, almost did) after Homer failed to jump Springfield Gorge in more than 16 years earlier on a skateboard in the episode "Bart the Daredevil".
In "Midnight Rx", the Canadian news refers to Homer as "Former American astronaut Homer Simpson".
In "Homer the Heretic" Moe declares himself to be a snake handler. In the episode "Eeny Teeny Maya Moe" (aired more than 16 years later) he says that he's a "snake handler but not an observant one".
In "Homer at the Bat", Smithers hires nine Major League Baseball players, including Mike Scioscia, to play against the team of the Shelbyville Nuclear Power Plant. All are prevented from playing by various misfortunes, with Scioscia getting radiation poisoning from working in the Springfield Nuclear Power Plant. Scioscia (now a manager for the Angels) appears in the episode "MoneyBART" (aired 18 years later); when Marge asks him about the radiation poisoning, he says it gave him "super managing powers".
In "Much Apu About Nothing", Selma gives her full name as "Selma Bouvier Terwilliger Hutz McClure", referencing her previous failed marriages to Sideshow Bob ("Black Widower") and Troy McClure ("A Fish Called Selma"). It was, however, something of a revelation that she was ever married to Lionel Hutz. Could be an Actor Allusion since Hutz and McClure were both voiced by Phil Hartman...
Maggie's shooting of Mr. Burns in season six is frequently alluded to in subsequent seasons, with the sheer implausibility of the incident often played up.
Marge: Homer, I don't want guns in my house! Don't you remember when Maggie shot Mr. Burns?
Homer: I thought Smithers did it?
Lisa (quietly, with disdain): That would have made a lot more sense...
In "Day Of Wine and D'ohsies" when Bart and Lisa are digging through the closet looking for a camera you can see many objects from past episodes including Homer's Mr. Plow jacket, Homer's Pin Pals uniform, a Scratchy robot head from Itchy & Scratchy Land, a Be Sharps album, Homer's boxing gloves from his career, Homer's cowboy hat from when he managed Lurleen Lumpkin, Homer's astronaut helmet, a Mr. Sparkle box, and the liquor bottle disguised as a civil war figurine.
"The Heartbroke Kid" features The Itchy & Scratchy Showcartoon "Kitty-Kill Condition" where Scratchy visits Itchy the cardiologist. The questionnaire asks if he has ever experienced certain events such as being churned into butter and hung by his intestines, which he has in past episodes.
The new version of the intro contains more allusions to older episodes than the old version. This is especially obvious in the scene with Marge at the supermarket: the old version had her buying generic products, in the new version she can be seen buying Tomacco Juice (the plant was created in "E-I-E-I-D'Oh") and Mr. Sparkle detergent (first seen in "In Marge We Trust"). Also, she reads a magazine with an "Absolut Krusty" vodka advert (the vodka appeared in "Bart Gets a Z").
"Politically Inept With Homer Simpson" featured a sign reading "SPRINGFIELD AIRPORT: Built For The Olympics We Didn't Get". In "The Old Man and the "C" Student" Springfield did apply for the Olympics.
This trope gets parodied in "Weekend at Burnsie's". Marge goes through a box of old clothes for a scarecrow, and little "Did You Know?"-style tips pop up for each one explaining which past episode they're referencing. The last one being "Grampa's hat from 'Who Shot Grampa's Hat?'"
In "Marge vs. the Monorail", Homer suggests that Bart change his name to "Homer Junior" so that the kids can call him "Hoju". In "Labor Pains", Homer plays with a baby named after him and calls him HoJu.
In The Spectacular Spider-Man episode "Reaction" Spider-Man swoops down to save a nerdy-looking guy and an attractive woman, leaving them tied up together and looking infatuated ("You can thank me later, dude.") In the later episode "Gangland" (set on Valentine's Day) we see that same girl happily accepting the guy's marriage proposal.
Kim Possible does this frequently, especially during its fourth season. Not terribly surprising that the creators would want to throw out these little "treats" for the rabid fans, considering that they're the reason the show has a fourth season.
Just to give one of the many examples, remember Drakken's mind controlling shampoo? Hank Perkins does when naming Drakken's cupcake business. He mentions that "We focused tested "Dr D's", but everyone associated it with shampoo for some reason." It appears again, when Drakken later distils it for a more concentrated mind-control formula. Also, Warmonga - the alien who believes "the Great Blue" will lead her people - comes to Earth and sides with Drakken after picking up the TV signal of the American Idol parody that he appeared on to promote the shampoo.
Any time Finn gets a new sword, he keeps that sword until he gets a new one. He had a gold sword until it's destroyed in "The Real You". In "Mystery Train" Finn picks up the Root Sword. He used this sword until he got the Steel Sword in "Morituri Te Salutamus", which is also used in "Still". In "Dad's Dungeon", Finn gets Joshua's Family Swordnote also known as the Demon Blood Sword. He has used this sword so far throughout the 4th season.
When Princess Bubblegum requests Finn and Jake to fight The Lich in "Mortal Folly", she gives Finn a gauntlet and tells him that it was used was by Billy to fight The Lich last time. Finn and Jake met Billy earlier in "His Hero."
Also from "Mortal Folly", we meet The Lich. We learned in "His Hero" from Billy The Hero's theme song that Billy defeated The Lich.
Who’s the greatest warrior ever?
A hero of renown? Who slayed an evil ocean?
Who cast the The Lich King down?
When Princess Bubblegum accidentally causes another zombie outbreak in "From Bad to Worse", she tells Finn that she still has her notes of how to make the antidote from the last time ("Slumber Party Panic").
Finn and Jake find a hatch that leads to an underground civilization that is made up of human-like people in "Susan Strong", and meet a woman they name Susan Strong. At the end of the episode all of her people retreat to the hatch. Susan comes back in "Beautopia" looking for help from Finn.
In "Mortal Folly", Princess Bubblegum gives Finn a pink sweater. He's wearing the same sweater in "Thank You."
In "The Creeps", Finn, Jake, Princess Bubblegum, Lumpy Space Princess, Cinnamon Bun, and Beemo are invited to a mysterious mansion where they end up trying to find out who's possessed and trying to kill everyone. When Cinnamon Bun dies and his skeleton is all that is left, everyone freaks out. Jake points out that this is just like the prank that he pulled on Finn in "Mystery Train".
In "Memory of a Memory", Finn and Jake go into Marceline's memories and find one of her dad eating some fries while a younger Marceline walks in. We found out in "It Came From The Nightosphere" that one of the main reasons Marceline hates her dad is that he ate her fries one time.
What am I to you? Am I a joke, your knight or your brother?
What am I to you? Do you look down on me 'cause I’m younger?
In "Holly Jolly Secrets Part II", after Finn, Jake and The Ice King bond over The Ice King's diary tapes, they decide to get together with friends every year when the weather gets cold and give each other presents. Among the many characters listed, the Royal Tart Toter is mentioned. He was first introduced in "The Other Tarts".
In "Hot to the Touch", Finn and Jake are looking through a bunch of junk in their house to build fireproof suits. They come across N.E.P.T.R., who Finn built in "What Is Life?" and wasn't seen after that. He claims to be the ultimate hide and seek champion at 15 months 4 days and 9 hours.
Also from "Hot to the Touch", FlamePrincess attacks the Goblin Kingdom. It's the same kingdom that Finn became king of in "The Silent King".
In "Dream Of Love", we see the pig who Finn rescued in "Apple Thief".
In "Return To The Nightosphere", Finn and Jake are stuck in the Nightosphere and try to find Marceline's dad, both of which were introduced in "It Came From The Nightosphere". In its followup episode, "Daddy's Little Monster", Finn uses the spell he learned from Marceline in "It Came From The Nightosphere" to get back to the Nightosphere.
During "In Your Footsteps", a bear chokes on some peanuts. After Finn fails at using the Heimlich Maneuver, he just whacks the bear with the Enchiridion, which Finn got in "The Enchiridion!" And then at the end we see the Snail being possessed by The Lich like in "Mortal Folly".
At the beginning of "Goliad", Princess Bubblegum mentions her Near Death Experience with the Lich from "Mortal Folly" when introducing Goliad. When Goliad is reading Finn's mind, Finn tries thinking of random things. One of the things that he thinks of is a baby Finn singing a goofy song, which is what Marceline saw in Finn's mind in "Memory of a Memory".
In "Beyond this Earthly Realm", Ice King mentions that he can see the spirit realm because of his wizard eyes, which was first brought up way back in "Mortal Recoil". And when Finn is looking for the Ice King, he says "Where'd you go Simon?" This is referring to the Ice King's real name from when he was a human, as learned in "Holly Jolly Secrets Part II."
"Son of Mars" brings back three characters: Magic Man (who first appeared in "Freak City"), Abraham Lincoln (from the original pilot), and Death (who first appeared in "Death in Bloom").
"Burning Low" has Princess Bubblegum call Morrow, who first appeared in "Death In Bloom". Princess Bubblegum also mentions that she was the one that told Flame Princess's dad to keep her locked up, which is how Jake found her in "Incendium". Jake also mentions how Princess Bubblegum hurt Finn, which also happened in "Incendium".
"Lady and Peebs" has the return of Ricardio, last seen in "Ricardio The Heart Guy."
"You Made Me!" features the return of Lemongrab, last seen in "Too Young."
"Ignition Point" has Finn and Jake sneaking into Flame Princess's room in the Fire Kingdom, and we see the glass that Flame Princess was held in that we saw in "Incendium."
"Reign of Gunters" has Ice King go to Wizard City, where he talks to Huntress Wizard, who participated in the tournament in "Wizard Battle."
"I Remember You" once again brings up Ice King's true identity of Simon Petrikov, and we see the Enchiridion in a newspaper clipping. We also see when Marceline first receives Hambo ( from Simon), which was first shown in "Memory of a Memory," and the Ice King consults the book Mind Games by Jay T. Dawgzone that was seen in "Reign of Gunters."
In a season one episode of Duckman, the title character causes a supercomputer to go haywire by presenting it with logical conundrums. A season later, despite the show's usual Negative Continuity, he spills coffee on another such machine destroying it, and remarks "Wow, me and supercomputers, huh?"
In an episode of Rugrats, Angelica arrives at Chuckie's house and Chaz answers the door to which Angelica remarks "Nice to see your hair grew back" referring to the episode "Chuckie's First Haircut" in which Chaz allowed Chuckie to cut most of his hair off and was shaved bald by the end of the episode.
Xiaolin Showdown is always bringing back Shen Gong Wu (magical artifacts) from previous episodes. It has become impossible for all but the most dedicated fans of the show to determine which Shen Gong Wu changed hands when.
During its latest season, Foster's Home for Imaginary Friends has been making numerous continuity nods to previous seasons, such as when Mac saw the "Rocket Wars" action figures he was forced to sell online in a previous episode when he was at a swap meet.
W.I.T.C.H. uses it to the extent that each episode continues on into the next, and keeps drawing aspects from as far back as the first episode. Objects, items, dialog quips, and plot points are always being referenced and built upon, to the point where if you miss an episode you can become entirely lost. As a result, the entire series is a continuity nod to itself.
Used so often Avatar: The Last Airbender that it could have its own archive. But to mention a few: The umbrella from "The Fortuneteller" is found in Appa's luggage by Sandbenders in "Appa's Lost Days". The eye-patch wearing Fire Nation commander from "Jet" shows up again in "The Cave of Two Lovers". The tsungi horn and ruby encrusted monkey statue Iroh buys in "The Waterbending Scroll" appear several times. And so on.
And of course the Cabbage Merchant, who appears in three very separate episodes and has his cabbage wagon destroyed in some way. Referenced in the playbill in "The Ember Island Players," as are many other things (the play itself is a recap - kinda - of the series thus far).
Sokka: His [Pu Wan Tin] sources include singing nomads, pirates, prisoners of war, and a surprisingly knowledgeable merchant of cabbage.
Let's not forget his posthumous nod in The Legend of Korra. When a certain prominent businessman is arrested as a suspect in support of the Equalists and exclaims "MY CABBAGE CORP!" as he is being led off in handcuffs from the company headquarters. And who is immortalized as a statue in front of said headquarters? None other than the cabbage merchant complete with cart.
"The Western Air Temple" features the Gaang bringing up every single villainous thing Zuko did in the first season, and Aang telling them who the Blue Spirit really was.
Roku mentions that learning waterbending was particularly difficult for him, calling back Aang's troubles with earthbending, his own "opposite" element.
The numerous Fenton devices that make up Danny Phantom, not to mention several previous events that are stated in later episodes, "Memory Blank" has quite a lot.
Also, keeping watch during the Grand Finale could result in spotting such obscure ghosts as the trio from the comic book convention, the Danny clones, the Greek Mythology ghosts, the mutants that attacked the summer camp, and even the ghost that the Guys in White used to bait Skulker.
Teen Titans has several of these, including the baby moth-monster that Beast Boy keeps as a pet and Starfire drinking mustard as if it were a normal drink.
The shorts New Teen Titans does this in the short "Blackfire's Babysitter" by not only referencing Blackfire's marriage, but revealing that she got saddled with four kids.note It's kinda implied it's the creature's offspring, not Blackfire's.
Several scenes in the junkyard show inventions from previous episodes (like the swinging-chair ride from "Eds-aggerate")
In "To Sir, With Ed", Eddy convinces his friends to come over by telling them a snake had snuck into his room. They show up wearing uniforms for "Eddy's Snake Bee Gone", which are just the "Eddy's Hive Bee Gone" outfits from "Pop Goes the Ed" with "Hive" crossed out and replaced with the word "Snake".
Then there's this exchange from "Stuck in Ed", where Eddy convinces Jimmy to help him think up a scam, and Edd brings up the events of the episode "Ed in a Half-Shell":
In "May I Have This Ed?" Ed produces a "lucky cheese chunk" he calls Sheldon Jr., a reference to his lucky cheese chunk Sheldon from "Thick as an Ed".
In "The Big Picture Show", Eddy's brother is shown wearing the same jersey and pants that Sarah and Jimmy wore in the episode "Ed, Pass It On", where they were masquerading as Eddy's brother.
In one episode, Eddy and Edd had to pull Ed out of Kevin's fridge, referencing an early episode where a sleepwalking Ed cleaned out every fridge in the culdesac.
The movie was full of continuity nods. References to previous episodes were shown throughout the episode.
The Venture Bros.: Watching through seasons 1 and 2 again after seeing season three, it's surprising just how much this series feeds back into itself like a giant ball of yarn. For instance, season 3 contains a complete explanation, ala Billy Quizboy flashback, as to why Phantom Limb...well, has invisible limbs. Season 2 sees about a third of an episode devoted to various main characters talking with each other about how Phantom Limb became the way he was, including Billy Quizboy in every story in one way or another (and saying that he lost a hand in one way or another in each). The post episode blurb (after the credits), had Dr. Venture asking Billy how he lost his hand, to which Billy responded he had no clue (again explained in the S3 episode — his memory was lost).
Anything mentioned in passing in the show's numerous Seinfeldian Conversations will eventually get a nod. In Season 2, The Monarch mentions that when he first met Dr. Girlfriend she had red streaks in her hair; in Season 3's flashbacks, she does. In Season 3, The Monarch brags that he once tricked Captain Sunshine into thinking that he was invulnerable; when Captain Sunshine finally makes his first appearance in Season 4, he still believes it - and by the way, Captain Sunshine was mentioned as far back as Season 1, when The Monarch tells his henchman to return "the charred remains of Wonderboy" to him; he eventually admits that he "kinda slew" the sidekick, and Captain Sunshine is seen to still be searching for a replacement.
Camp Lazlo had a few of these. On the episode "Hello Dolly", Edward has a Barbie-esque doll named Veronica, who makes cameos later on "No Place Like Gnome" and "Beans in Toyland". On another episode, the campers start the Camp Kidney radio station back up, and Lazlo is shown in the station at the start of the episode "Samson Needs a Hug".
Family Guy has a few, but the most obvious one is when Brian throws a rock at Peter's head in the episode "The Fat Guy Strangler" (instead of hitting the serial killer who had just let him go). When Peter chastises him he explains that he was aiming for Peter in the first place as belated revenge for a gag in the episode "To Love and Die in Dixie", which aired four years ago (specifically, not rolling down the window of their mock General Lee they had built before encouraging Brian to jump in through the window).
This seems to be a Running Gag with Brian. The episode "Airport '07" had Stewie about to take a drink from a cup that Peter had actually been using to spit his tobacco in. Brian started to warn Stewie about this, but then remembers a bit from an earlier episode, "Patriot Games", where Stewie beat him to a bloody pulp, and decides against telling Stewie anything (with unfortunate results for Stewie).
Cleveland gets quite a few nods in this direction, particularly in regards to the Running Gag of him always being in his upstairs bathtub when Peter's cutaway hi-jinks destroy part of his house:
Cleveland: I need to stop taking baths during Peter's shenanigans.
After Cleveland leaves Quahog, this same gag is revisited with Peter commenting about the empty tub: "Oh, that's right. Cleveland moved."
In the episode where Meg gets a legitimate boyfriend, every previous (and still living) character gathers at once to witness the 'miracle', including Cleveland who comments "This was worth the 500 mile trip!" (as his new show is set in Virginia.)
The Kool-Aid Man crashing into the courthouse saying his "Oh, Yeah!" Catchphrase during the first episode "Death Has A Shadow" was revisited again in the episode "Stewie Kills Lois".
Judge: Okay, can I ask everyone to please stop saying "Oh, no!" in this courtroom? 'Cause the fucking Kool-Aid guy's gonna keep showing up! Thank you.
In the episode "Deep Throats", Meg is dating Mayor West. In "Tiegs for Two" (aired five years later) they walk into a restaurant together, with Meg saying "it still happens sometimes".
In an early episode of The Tick, the villain Chairface Chippendale tries to write his name on the moon, but only gets three letters in. For the remainder of the series, every time the moon appears the letters "CHA" are visible on its face. The lunar abuse continues when the Tick goes to the moon to erase the letters, but only removes the "C" before being blasted into deep space. He meets a Galactus-pastiche and convinces him to spare Earth by snacking on the moon instead. A set of bitemarks accompanies the "HA" forever after.
It may be worth noting that this is something that shows up in the comics, too, as to this day, as the comics are still being published, every single panel with the moon in it will have "CHA" written on its surface.
In the pilot episode of Aqua Teen Hunger Force, a large rabbit robot (named the Rabbot) put a large hole in the side of Dr Weird's laboratory/asylum. The Rabbot-shaped imprint remained on the wall of his lab for the rest of the first two seasons and the movie, even playing a major role for how his creations escape or how leprechauns break in to steal his inventions.
The Aqua Teens' television is destroyed in nearly every episode. However, in "The Cloning" after Shake shoots the television with a flaming arrow, Frylock complains about everyone he destroyed throughout the series.
In an episode of King of the Hill, everyone is concerned about Hank's anger problem after he accidentally cuts off Dale's finger. Later on, when they go to a funeral of an anger management classmate of Hank's who had a severe anger problem, although dead, he still has an angry scowl, and Peggy says to Hank "that's what you look like when you sleep—" and Hank thinks back at all the mean things he's ever done shown in a montage of clips from many past episodes highlighting Hank's most aggressive and violent moments.
Nearly every time someone is shown reading a book it's always A Dinner of Onions, which is from the episode where Peggy buys a bookstore.
In a Season 5 episode, Buck wishes his son was like Bobby after he takes Bobby under his wing. Hank is surprised that Buck has a son, and Buck responds by saying that his name is "Ray Roy or something like that" and he lives in another state. In Season 13, when Buck meets his son in Memphis, he doesn't remember his name and calls him Ray Roy.
Invader Zim: One scene in "Battle of the Planets" has the Almighty Tallest recalling some of Zim's more humorous transmissions. The scene they were recalling — "Remember that one time... when he called up... and he was completely covered with meat?" — refers to the episode "Germs", which ended with Zim using germ-proof napkins to keep from contracting the titular threat.
In "Dib's Wonderful Life of Doom", Zim makes Dib hallucinate that aliens from the planet Meekrob give him special powers. Either a month before or minutes afterwards, depending on whether you go by airdate or production code order, Invader Tenn is said to be stationed on Meekrob in the episode "Megadoomer".
This episode also mentions that the Massive's weakness is its snack storage pods. In "Backseat Drivers From Beyond The Stars", the Resisty's attack on the Massive is to steal those snack supplies.
In "Battle of the Planets", Skoodge is seemingly killed when he is fired out of a cannon straight at Blorch, the Planet of the Slaughtering Rat People (It Makes Sense in Context.) However, he reappears in "Hobo 13", and rather than ignore the problem altogether Zim actually calls him on this. Skoodge Hand Waves it by saying he got better. This is, in itself, a nod to the first episode, where Skoodge is initially sent to conquer (or rather, be slaughtered by for being so short) the Rat People.
Dib goes to Zim for help in "Gaz: Taster of Pork", and Zim—being Zim—laughs at him and then turns him away. Dib mentions that he helped Zim when they were turning into baloney loaves, way back in "Bolognius Maximus". This messes with Negative Continuity a bit, as the episode ended with them failing to find a cure, and being trapped as baloney loaves hiding in an abandoned house surrounded by feral dogs.
In "The Chronicles of Meap", Dr. Doofenshmirtz mentions having to stand in for his family's lawn gnome, as he mentioned previously in "Lawn Gnome Beach Party of Terror!", and even tells Perry the Platypus "You remember that backstory..."
In "De Plane! De Plane!", Candace gets jealous when she sees Jeremy talking to another girl at the pool party, and Stacy references the events of "S'Winter" in warning her not to get carried away.
Stacy: Slow down! Remember how you thought Jeremy was with a girl, and she wasn't even a she? And then you ended up on that snowboard, all crazy and out-of-control?
In "Invasion of the Ferb-Snatchers", Candace freaks out when she thinks Ferb has been replaced by an alien doppelganger, and goes to hide in the panic room first seen in "I, Brobot".
In "We Call It Maze", Isabella's mentoree Melissa recognizes Candace as the girl who earned fifty accomplishment patches in one day, as seen in the episode "Fireside Girls Jamboree".
In "Brain Drain", Doofenshmirtz notes that his "De-Volition-inator" is not to be confused with his "De-Evolution-inator from a few schemes back", as seen in "Phineas and Ferb's Hawaiian Vacation".
Taken to the extreme in "Rollercoaster: The Musical!" "Mom, Look!" has Candace rattling off a list of the boys' previous schemes, and the finale "Carpe Diem" features almost every character in the whole show at that point.
In "Bullseye," Linda mentions that lip synching isn't her only talent.
In Code Lyoko, all of the characters have their Limited Wardrobe overhauled right in the middle of William's possession, where he, being trapped inside the supercomputer, obviously would not have time to change clothes. His clone gets a new outfit instead...but when he's released from the supercomputer, he's still wearing his old outfit, as if to say "see, we remembered!" It might've been more effective if he hadn't come out of the computer, clothes changed, a few episodes earlier.
A similar nod with the flashback of how Aelita got virtualized for the first time years ago at the end of Season 2; she's seen wearing (logically) the same clothes as with her first virtualization at the end of Season 1. (Earlier flashbacks tends to have her wearing her current Limited Wardrobe clothes, but since those are basically Aelita hallucinating, they are a bit more subjective.)
In Stroker and Hoop, the private detective duo puts out a series of billboard advertisements that place them as the subject to ridicule by most who see them due to a typo. These billboards show up in a few episodes, even going as far as to getting them in trouble with a drug king-pin, who identifies them by the glaring billboards.
"Dammit, we still have those things?"
Every episode has in some way a background character with the same voice but never commented on. In the Series Finale they're captured by a guy seeking revenge against them but they don't recognize who He is. Turns out the voice was the same guy every time and goes on about Stoker and Hoop ruined His life, accompanied by a montage of His appearances.
In Winx Club season four, the final battle against the Wizards of Black Circle happens in the Omega Dimension, the ice prison Valtor escaped from in season three.
When the girls visit Gardenia in S2, Bloom sees auras over guards at a nightclub, in reference to Bloom seeing auras over many people in the S1 episode in which she visits her home on spring break. 4Kids cuts that part out. 11:21 in this video.
From the same episode, Vanessa talks to Bloom about having to testify on trial against the guys who burned down her flower shop back in the S1 episode in question (in fact, that's the whole reason she's headed to Gardenia in this episode), but in the 4Kids dub, Vanessa talks about trying to get extra help for the flower shop. (10:08 in the above video.)
In the episode "New Leaf", you can briefly see the robot version of Mr. Krabs from "Imitation Krabs" in a pile with Plankton's other "failed attempts at the formula". It also reappears briefly in "Spy Buddies".
A rather subtle and interesting one in "Back to the Past". Spongebob and Patrick go back in time and see past versions of Mermaidman and Barnacle Boy using a giant can of tartar sauce to trap Man Ray. Now think back to Man Ray's first appearance in "Mermaidman and Barnacleboy III" where he is shown to be frozen in solid tartar sauce.
There was a rather pointless one in the sea whelk episode. When Spongebob learns that the whelks are sick, he exclaims, "I know! I had the Suds! I know what to do!" This was followed by something completely unrelated to said episode where he had the suds.
When Spongebob and Patrick plan to do something nice for Squidward in "Enchanted Tiki Dreams", Patrick mistakenly says "impressed" when he means "depressed" much like he did In "Fools In April". Also Spongebob's "abrasive side" in the episode "The Abrasive Side" makes an offhand reference to Mr. Krabs keeping his money in his mattress, as was established in "The Lost Mattress".
Ruby-Spears' Mega Man was generally self-contained, but during an Enemy Mine situation, Wily comments on how nice it is that he and Dr. Light are working together again. This happened in both the games' backstory and the first episode of the show.
An episode of Jimmy Two-Shoes had Jimmy, Beezy, and Heloise singing a song in order to ger Samy to be distracted by his dreams of stardom in order to stop Lucius' plot. In later episodes, Jimmy's favorite band Runny & the Nosebleeds perform the song.
Some of the ghosts busted by The Real Ghostbusters can be seen in the containment unit in later episodes of the show.
G.I. Joe: Renegades will occasionally reference locations, incidents, and characters from previous episodes.
The Courage the Cowardly Dog episode "Mega Muriel the Magnificent" mentioned events that have happened in four previous episodes, such as "A Night at the Katz Motel", "Freaky Fred", "Queen of the Black Puddle", and "Snowman Cometh",
In "Nano of the North", the Professor's car has a license plate "PWR PROF", a reference to an earlier episode of the same name where he called himself "Power Prof" while wearing Powered Armor to fight crime with the Girls.
In "Reeking Havoc," Blossom gets a giant match with which to battle a giant fart monster. When Buttercup asks where she got it, Blossom replies "Same place I got the giant jar, silly. Season one, episode two...remember?" (She refers to "Insect Inside," where she used a giant jar to encase Roach Coach.)
The Smurfs: in the cartoon special "My Smurfy Valentine", Chlorhydris the evil witch spoke of Azrael having "royal blood in his veins" — which interestingly turns out to be true in Season 9's "Mummy Dearest" when the Smurfs meet his distant ancestor, the cat pharaoh Azra
Three episodes — "The Smurfbox Derby", "Handy's Window Vision", and "Skyscraper Smurfs" — are continuity nods to things Dreamy has seen in a dream in "Gargamel's Miss-Fortune".
A vision of Smurfette recalls to Dreamy's attention three past adventures he had been on — "The Last Smurfberry", "The Astro Smurf", and "Handy's Kite" — in "The Smurf Who Would Be King".
And also in the sequel stories, such as "The Astro Smurf" being remembered in "Dreamy's Pen Pals", and Smurfette's origin in "The Smurfette" being remembered in "Smurfette Unmade".
A second season episode of Regular Show is called "A Bunch of Baby Ducks". This comes from a line in the first aired episode, when Rigby explains he may have used The Power (a magic keyboard) to send things to the moon.
Rigby: A buncha baby ducks, send 'em to the moon! Soda machine that doesn't work, send 'em to the moon!
Their latest meeting with Todd ("Snitchers") results in them having to testify against him in court. Todd's attorney then attempts to discredit them based on their stupidity, referencing "Frog Baseball" and "Held Back".
One Robot Chicken sketch was a promotional video for Cobra Command, and included Cobra soldiers being lectured to do things in a needlessly complicated and snake-themed manner, for instance instead of using a gas grenade they would use a gas-shooting robot snake in a wicker basket. A later episode in the form of a documentary about the battle over Cobra's Weather Control Machine ended with Duke saying Cobra Commander was captured then immediately broken out by Zartan using "a wicker basket thing and a remote control sneaker or something".
My Little Pony: Friendship Is Magic has tons and tons of continuity nods, subtle or overt. Some of the most prominent ones are Rainbow Dash being shown reading a "Daring Do" book every once in a while after the episode "Read it and Weep" and Pinkie's "Pinkie Sense" returning in Season 2's "The Mysterious Mare-Do-Well" and "It's About Time" from Season 1's "Feeling Pinkie Keen".
Possibly the most subtle is from the season 3 finale, Magical Mystery Cure. Check how long it takes for Rainbow Dash to clear the skies after Rarity has been screwing up Rainbow Dash's job.
One definitely worth mentioning is the insane pony that barks like a dog in It's About Time. Fast-forward to Just For Sidekicks in Season 3, and we catch a glimpse of the same pony who has since recovered a great deal and is on her way to being fully sane.
In A Johnny Bravo Christmas, Johnny meets Santa and mentions the episode where he knocked him out thinking he was a burglar. Santa doesn't recall that moment, but gives Johnny a punch in the face for it.
In the American Dad! episode "Love, AD Style", when Francine tells Stan that Roger had shot Haley, Stan recalls the time she shot him in "Haylias" and when he shot her in "Hurricane!".
In "Stan's Best Friend", Francine mentions their two previous dogs, Thor in the first episode and Fussy from "Not Particularly Desperate Housewife". Stan says he doesn't remember them and assumes they were dreams.
In "Killer Vacation", when Francine survives a bullet because of a hip-hop necklace, Stan says "Thanks once again, hip-hop", recalling a line from "Finances With Wolves".
In "Buck, Wild", Steve tries to show Stan how much of a man he is by showing him a PowerPoint presentation on some of his past "manly" exploits, which include scenes from "The Magnificent Steven", "Toy Whorey" and "Man in the Moonbounce".
In Stitch! The Movie, Pleakley asks Cobra "What do we do?", to which Cobra responds "Hope for another miracle." This mirrors Jumba's response when Nani asked the same question in the first movie.
In the Ben 10 universe, Ben acquired three aliens based off of Universal monsters (a mummy, werewolf, and Frankenstein's monster) which never show up in a canon appearance outside of the four-episode story arc in which they debuted and never got an official name. In the "Frogs of War" special, the randomizer on Ben's Omnitrix turns him into the mummy alien. Ben remarks that he can't even remember what "this guy" can do, which means he probably never used it at all outside of its debut story arc (which seems to be how the show's writers intended those characters to appear).
Danger Mouse has a couple. "The Return of Count Duckula" has a slightly altered recounting of DM's first encounter with the count, "The Clock Strikes Back" has the time-traveling clock from "The Hickory Dickory Dock Dilemma" returning (as a vessel for a megalomaniacal magician), and "Penfold Transformed" has Greenback acknowledging the failure of his duplicator from "Tiptoe Through The Penfolds."
VeggieTales has one of these in the 2009 Christmas Special (a retelling of the story of Saint Nicholas): when Bob is upside-down in the snow (which is in the middle of the desert thanks to Larry's imagination), he says "You roll your dice, you move your mice." This references the 1996 Christmas Special, in which he wanted to play Mousetrap instead of go sledding. After a sledding accident puts him upside-down in the snow, he says the same line.