In the episode "Love, AD Style", when Francine tells Stan that Roger shot Hayley, Stan recalls the time Hayley shot him in "Haylias" and when he shot Francine in "Hurricane!".
In "Stan's Best Friend", Francine mentions their two previous dogs, Thor from 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 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 "Big Blowout", Mrs. Felt mentions how Mookie visited their place once, which happened in "Sick Day".
In "Worm Boy", when Joey falls ill from silkworm goo coursing through his veins, AP assumes that he had either gotten the flu or been bitten by a tick again, which happened in "Sick Day" and "Tick'd Off" respectively.
The Private Proton costume Mookie gave to Joey in the episode of the same name is still in Mookie's room in "Parallel Puppet".
In "Claude Returns", Claude uses the super sticky glue gun that Joey used to halt Lacer in "These Shoes", which he and Mookie retrieved from Captain Atomic's Invisi-Shed (also seen in "These Shoes").
AP's biography that first appears in "Don't Track the Mudman" reappears "Lacer Takes Over", where in the latter episode Joey comments that he thought AP stopped writing it a while back. Likewise, Travis (a Ken doll expy owned by Joey's sister) from "Lacer Takes Over" originally appeared in "Big Blowout" as a quick joke.
The villains seen in the Mega City Maximum Security Prison for the Criminally Motivated in "The Big Shift" include a number of generic crooks used in previous episodes as well as some lesser Rogues Gallery members who had been arrested in previous episodes.
Mintenberg's hang gliding mice from "AP vs. Disastro II" are still with him in "Brawl-For-All". Not to mention Joey and AP scold him for having broken his promise to stop conducting experiments in the former episode, referencing what they told him in "Something Chicken".
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.
Also 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.
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".
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 of Omniverse, 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).
While Big Hero 6: The Series has the occasional nod, "Hiro the Villain" is completely full of them. Most of them involve the subplot of Megan Cruz investigating the true identities of Big Hero 6, interviewing characters connected to them and tying together previous events from the series and the movie. The main plot also calls back the plot of "Food Fight", a very early episode from season one.
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".
In Central Park, Season 1 "A Fish Called Snakehead", when Paige brings up Bitsy wanting to buy Central Park and the mayor helping her, Owen starts anxiously rubbing his hands on his shorts like he did in "Dog Spray Afternoon". And Cole and Helen are still on good terms from the same episode.
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.
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.)
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". Kind of odd considering the show has no continuity.
On Creative Galaxy, this is downplayed - when Arty visits Cooktopia in the second story of the Easter episode, having been there in the first story, Chef Zesty makes a passing comment about him being "back so soon."
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."
The numerous Fenton devices, 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.
In "Hereafter", the trunk of the car Superman is driving has a box of the energy bars Flash was hawking in "Eclipsed", and Deadshot smuggled a gun into the watchtower by hiding the parts in wrappers of the same bars in "Task Force X." "Flash and Substance" featured a cardboard cutout of him in his apartment, still advertising the same energy bars, which is destroyed when it startles Orion.
Flash: [Bummed out] Dude, that was my last standee.
In "Clash", Superman brings up deodorant advertisement deals in his lecture to Captain Marvel—the way Flash covers his face indicates that he takes this as a dig against his energy bar commercials.
In "Hearts and Minds", Katma Tui brings up that John Stewart sent Green Lantern Kyle Rayner to Oa for training after receiving his ring, accounting for his absence since his introduction in "In Brightest Day..." in Superman: The Animated Series.
In "Question Authority", Huntress uses Jimmy Olsen's signal watch to call Superman; the watch first appeared in "Superman's Pal" in Superman: The Animated Series.
Another reference appears in "Only a Dream" when, during his nightmare, Superman calls Jimmy "my pal."
In part two of "The Once and Future Thing," Green Lantern alludes to a past team-up with Static. This references "Fallen Hero", an episode of Static Shock that Green Lantern guest-starred in.
Lex Luthor has cancer from manipulating kryptonite for years. The first mention of the disease was in Superman: The Animated Series, when the Joker stole a jade dragon statue. Every past owner fell ill and died, because it actually was made of kryptonite.
The entire Cadmus Arc from 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, (a.k.a. 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!) Then again, considering the kind of relationship the Joker has with his girlfriend...
When the inmates of Arkham Asylum puts Batman on trial, it's one big nod to the first appearances of each of the villains who "testify" against him.
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 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.
In Defenders of the Earth, it is established early on that Kshin is forbidden to touch Mandrake's sorcery books; this forms the basis for the plot of the third episode in the series. In a later episode, Kshin has to work on a book report for school and Mandrake, before departing on an expedition to Morocco with the rest of the Defenders, reminds him that his sorcery books are off limits.
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?"
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".
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 "Fistful of Ed", when Eddy mocks Jimmy during his "fight" with Edd, he recalls the time that he and the other Eds turned Jimmy into a sumo wrestler in "One Size Fits Ed".
In Ed, Edd n Eddy's 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 him.
The movie was full of continuity nods. References to previous episodes were shown throughout it.
In "Run For Your Ed", Eddy and Edd had to pull Ed out of Kevin's fridge. Edd wonders if Ed has been sleepwalking again in reference to "A Glass of Warm Ed", where a sleepwalking Ed cleaned out every fridge in the culdesac.
In "Robbin' Ed", when Eddy (as Professor Scam) captures Jonny (as Captain Melonhead), he prepares to dunk him in Ed's bathtub full of gravy from "Mirror, Mirror on the Ed".
In "No Speak Da Ed", when Rolf believes that Ed's wolf-themed gifts mean he's in league with the rustlers that terrorized Rolf's livestock as a child, Eddy mocks him by asking if the worst he can do is stick eels down their pants again, referencing the ending of "Dueling Eds".
Battle for Sunstar, the last episode of Ewoks, where Dr. Raygar tries to steal the Sunstar and overthrow emperor Palpatine implies that the events of the series took place sometime between A New Hope and Ewok Adventures.
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".
The latest season 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.
The weird photo of Frankie on her driver's licence from "Bus the Two of Us" is the same picture from Bloo's smear campaign in "Setting a President".
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 "All the Presidents' Heads", Farnsworth chides Fry for changing history by saying "You've really screwed the granny this time!", again referencing "Roswell That Ends Well".
In "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.
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.
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.
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.
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.
In one episode 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.
At the beginning of the Let's Go Luna! episode "She is the Moon of Moons", Andy, Carmen, and Leo look at photos of their previous travels. The photos refer to the events of the episodes "Hola Mariachi" (where the kids are in Mexico and try to find a mariachi band to fill in for their current band when they get the hiccups), "You Froze My Shorts" (where the gang helps Leo to warm up on a winter day in Russia), and "Aren't We a Pair?" (where Carmen meets a girl in Egypt who shares similar interests with her).
The Loud House: In "L is for Love", Mrs. Loud tells her kids the story of how she met her husband while working as a crossing guard, which she previously mentioned in "Cover Girls". In addition, Mr. Loud is shown to have spent a semester in England (like he said in "Study Muffin") and Mrs. Loud is wearing what would become Lori's tank top (which Lori said in "Hand-Me-Downer") on their first date.
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.
In "Applebuck Season", Applejack explains to Twilight that her huge family was only in town for the Sun Festival (the pilot episode), which is why she's the only one left on her farm that's strong enough to harvest the orchard after Big Macintosh hurts his back.
During "Dragonshy", Rainbow Dash mentions the ferocious manticore that Fluttershy befriended during the pilot episode.
In "Suited For Success", Rarity is creating the dresses that she and her friends will wear for the Grand Galloping Gala announced in episode 3. Some of these dresses can be seen in later episodes (Rarity wears hers at the beginning of "A Bird In The Hoof"). The Gala itself is the season finale.
In "Green Isn't Your Color", Rarity states that she "vants to be alone" when she is upset over Fluttershy's popularity. She said the same thing in "Suited For Success", after a failed fashion show made her a laughingstock in Ponyville. While sorting through dresses for Fluttershy to try on, she tosses aside several outfits which appeared in prior episodes.
In the first episode, one of the dresses Rarity has Twilight try is a Statue of Liberty outfit. In "The Cutie Mark Chronicles", we learn that a pony version of the Statue of Liberty actually exists in their world.
The straight-haired version of Pinkie Pie seen in "Cutie Mark Chronicles" reappears in "Party of One" when Pinkie Pie convinces herself everypony is avoiding her because they no longer like her parties.
Also briefly in "The Best Night Ever" as she tries to straighten it for the Gala. But because she's happy, it goes poofy again immediately.
And once again in "Magical Mystery Cure", where she is miserable due to being stuck with Applejack's destiny.
And Zecora, debuting in "Bridle Gossip", establishing the context for her cameo one episode later in "Swarm of the Century". She's also mentioned (but not seen) in "Stare Master"; visiting her for tea is Twilight's reason for going into the Everfree Forest that night.
In "Boast Busters", Snips and Snails are shown to be awe-struck by Trixie's magic skills. In "The Show Stoppers", they attempt to stage a magic show of their own.
"The Return Of Harmony, Part 2" features two short segments with several clips taken from previous episodes, when Twilight and Applejack's memories are being restored.
Another nod to "Bridle Gossip" was made in the Season 2 episode "The Cutie Pox". Seems Zecora hasn't forgotten her initial treatment by Ponyville's inhabitants.
In "The Mysterious Mare Do Well" Pinkie Pie says she used her Pinkie Sense (from "Feeling Pinkie Keen") to save the construction workers. She then immediately gets a twitch and pushes Rainbow Dash out of the way of a falling flower-pot. Pinkie Sense then gets brought up again during "It's About Time", where Pinkie is doing a fortune telling routine. Twilight asks Pinkie to use her Pinkie sense to predict the coming disaster, but Pinkie says she doesn't use her Pinkie sense for that. Pinkie then immediately gets a twitch and a flower-pot falls on Twilight.
All of them are listed on the recap page for any episode.
In "The Return of Harmony, Part 1", Sweetie Belle corrects Apple Bloom by telling her "victoryful" isn't a word. Scootaloo asks "What are you, a dictionary?" In "Family Appreciation Day", Apple Bloom rapidly lists synonyms for "embarrassed", prompting Scootaloo to ask her the same question (the same recording was even used).
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.
Derpy, despite not competing herself, is still part of Ponyville's delegation, as shown in "Rainbow Falls", probably as an emergency replacement in case it's necessary.
Pinkie Pie is the only person in the audience who enjoys Spike's rendition of the Cloudsdale anthem. She even shouts "Nailed it!" at the end. This is an inverse of the scene from "Over a Barrel", where Spike was the only person in the audience to enjoy Pinkie's "You Gotta Care, You Gotta Share" song.
Tirek's brother Scorpan is shown in the story flashback to go off with a unicorn wizard; though the name is not said, this is clearly meant to be Star Swirl the Bearded, based on similar imagery from "Three's a Crowd".
Once more, Fluttershy is the only one of the Mane Six to find Discord's antics rather amusing.
All the ponies who gave the keys to the Mane 6 over the season are shown as the rainbow flies through Equestria.
This is the second time this season we've seen Shining Armor in uniform, and the first time since "A Canterlot Wedding" that he's been shown performing his duties as Captain of the Royal Guard.
Cerberus' escape from Tartarus in "It's About Time" is directly mentioned, and cited as the most likely reason Tirek was able to escape.
The infamous "The Mean 6" has Pinkie recall to Mean!Twilight the climax of "Twilight's Kingdom - Part 2" from when they defeated Tirek: the Rainbow Power, the Castle of Friendship rising out of the ground, and the song, "Let the Rainbow Remind You", in that order.
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 "Keen on Keane" the Professor references the episode "Cat Man Do" when he mentions a really bad experience that he had with a cat, which made him jump off a building.
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.)
In "Mindy Pet-Sits", Mindy comes across the meteorite that she found in "Mindy's Meteorite Stand".
In several episode, the kids point out how they have already been to certain planets.
Jet builds a robot in the episode "Jet 2". In later episodes, the robot can be seen around the children.
Some in "Mindy Turns Five":
Similar to how Mindy was jumping on the bounce house in "Mindy's Moon Bounce House", she is seen jumping on it again, and is singing a song that is sung to the tune of "My Name is Mindy".
Before the gang gets back to Earth, they look at and describe Earth like they looked at and described it in the episode "Tiny Blue Dot".
The gang goes to Pluto a second time, and they sing "Dear Little Frozen Pluto" again.
The squirrel that ate Mitchell's jellybeans in "Earth Camping" makes several more appearances throughout the show, in "The Mindysphere", "Space Race", "Jet Shrinks the Kids", and in "Mindy's Bedtime", he is finally given a name: Floyd.
In "Galileo, Galileo!", Mindy unintentionally makes Sean nervous to do his report on Galileo, and while doing so, Sean imagines himself on the exact same stage that the kids did their Christmas pageant in "Holidays in Boxwood Terrace", which is where Sean got nervous to perform his song in front of everyone.
In "Total Eclipse Block Party", Sean and Sydney initially won't let Jet perform on the DSA stage for fear that he will sing and use his Bortronian robot arms, which is a reference to the events of "My Fair Jet".
"Racing on Sunshine" takes place one year after "Kid-Kart Derby".
"Asteroid Belt Space Race" is about the race Celery and Zucchini promised to have at the end of "Space Race".
Sean says he sings in the shower in "Potatoes on Mars", which was previously mentioned in "Holidays in Boxwood Terrace".
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!
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".
After Tommy asks Chuckie "Have I ever got us lost forever?" in "The Big Flush" Chuckie cites incidents from past episodes:
Chuckie: How bout the time you took us down in the basement and I got stuck in the mattress? And the time you got us locked in that toy store! And the time you made us go through that mirror into Mirrorland! And the time we chased after that wedding cake! The time we got lost in the museum!...
In "Give and Take" 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.
The opening scene of "Finsterella" shows the adults fawning over Chuckie learning to talk, his only word being "No", referencing a memorable moment from Rugrats in Paris.
The episode "Ransom Of Cynthia" introduces a plot point involving the adults trying to cut down on the babies' sugar intake and protect their teeth. This same plot is revisited soon after in "Angelica Orders Out".
Melville, the pet bug briefly owned by Chuckie before its death in "I Remember Melville", is referenced when the babies are talking about their pets in the episode "Share And Share A Spike."
The All Grown Up! episode "The Curse of Reptar" has Chuckie remembering how important Reptar was to the gang when they were babies, saying that he was their first movie ("At the Movies"), cereal ("Incident in Aisle Seven") and ice show ("Reptar on Ice").
Shimmer and Shine: In "Double Trouble", when Leah and her genies find out the gem they're looking for already belongs to someone else, they wonder what they'll have to do to get it and guess things like solving a riddle and winning a race. In "Zoom Zahramay", a gem was offered as a prize to the winner of a flying carpet race, and they had to solve a riddle to win the gem featured in "Mermaid Mayhem".
In many episodes there are small bits in the background which recall earlier episodes. Most notable is probably the giant stone head that Mr. Burns gave the family in the final episode of Season 2, and has been seen in the Simpsons' basement ever since. Any shot of a closet will invariably reveal it to be packed with props from former episodes. On several occasions there have been conveniently placed photo walls in the Simpsons' house, revealing past adventures; in "Homer's Enemy" Frank Grimes reacted in complete disbelief when he saw photos of Homer winning a Grammy award and in outer space, for instance. In the Season 18 episode "You Kent Always Say What You Want" Homer shows off his wall of "Wall of Casual Acquaintances Who Came to Stay for a While" featuring portraits of 10 characters who had lived briefly with the Simpsons. However, despite such things, The Simpsons generally has Negative Continuity. It plays differing continuities against each other, like the episode where Homer took Ned Flanders' roof to use as a snowplow, and Flanders complained that Homer used to be a professional snowplower and surely still had the equipment. Homer reacts obliviously, humming the Mr. Plow 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?
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 more than 16 years earlier on a skateboard in the episode "Bart the Daredevil".
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 "MoneyBART" (aired 18 years later); when Marge asks him about the radiation poisoning, he says it gave him "super managing powers".
"Viva Ned Flanders" opens up with the Monty Burns Casino, which was built when the town legalized gambling in "$pringfield", being demolished. Marge observes that it's curious how they forgot all about the place a week after it opened. Lisa adds it's even weirder how they bothered to keep it when they moved the town, which happened at the end of "Trash of the Titans".
In "HOMR", Homer is revealed to have been a genius as a child before he shoved a crayon up his nose. Many, many seasons later, during a growing-up montage (itself a Shout-Out to Amélie), in one photo his grade is an A+; the next, it's an F- -.
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'").
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 (the non-existent) '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.
Any episode featuring Japanese characters will generally feature a moment in which they remark on how Homer resembles Mr. Sparkle, for instance, Comic Book Guy's father-in-law in "Married to the Blob".
The plot of the 12th episode of Season 26, "The Musk Who Fell to Earth," has Elon Musk hired by Springfield Nuclear Power Plant, causing it to almost go bankrupt. For the remainder of Season 26, this event is often referred to, as the Plant struggles to recoup its losses.
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".
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 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 "Stunning and Brave", the school's new PC Principal rattles off some of the hateful things the townspeople have said and done over the years, referencing events from "City Sushi", "Child Abduction is Not Funny", "The Cissy" and "The Return of Chef"
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.
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 SpongeBobs "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".
Since Hillenburg had returned to the show, there had been a lot of continuity to older episodes.
Earlier episodes had some continuity, like Spongebob meeting Sandy for the first time and Mermaid Man and Barnacle Boy remembering Spongebob from previous encounters.
Hera Syndulla, the Twi'lek pilot and captain of the Ghost, is the daughter of Cham Syndulla, the leader of the Twi'lek freedom fighters during the Separatist occupation of Ryloth. Cham was also one of the main characters in Lords of the Sith, which mentions Hera in passing a few times.
The visor-shape of Sabine's helmet has the same owl-face design as Bo-Katan's helmet, who was the second-in-command of Death Watch. As it turns out, she is descended from House Vizsla through her mother, who was a member of Death Watch.
Concept art also shows that Sabine has created graffiti-portraits of Cad Bane and Embo on the walls of the Ghost.
In the pilot movie, Grint and Aresko harass a Lothal citizen over jogan fruit, which were first introduced in Star Wars: The Clone Wars.
The Empire makes the same mistake they did in Return of the Jedi, jamming the Rebels before the trap was fully sprung.
In "Droids in Distress", Bail Organa checks R2-D2 to find more about the rebels while on an Alderaan Diplomatic ship, a nod to how his adopted daughter Leia uploads a message to R2-D2 during A New Hope while her Alderaan Diplomatic ship is being boarded by the Empire.
"Breaking Ranks" features Kanan and Hera stopping a Kyber crystal from being delivered to the Empire. In the Star Wars: The Clone Wars "Crystal Crisis on Utapau" story reels, a giant Kyber crystal was featured as the MacGuffin that Obi-Wan and Anakin tries to prevent from falling into Grievous's hands, and it's heavily implied to be the power source of the Death Star's laser.
The episode also resembles "Death Trap" from The Clone Wars in that a boy who's more skilled than his peers (Boba Fett/Ezra Bridger) infiltrates the army ranks as a cadet.
Yoda appears to Ezra as a swarm of fireflies; Qui-Gon Jinn appeared to Yoda the same way in the Yoda arc for Star Wars: The Clone Wars.
When Kanan questions how Yoda can commune with him, he responds with "I am here because you are here". Qui-Gon said the same thing to Obi-Wan on Mortis in Star Wars: The Clone Wars, when Obi-Wan questioned Qui-Gon's presence.
The artwork in Hera's cabin resembles Twi'lek art seen in the Ryloth arc of The Clone Wars
In "Fire Across the Galaxy", the rebels steal an Imperial Transport from the same TIE Fighter landing field Sabine raided in the "Art Attack" short. The stormtroopers even remember her, saying "the artist is back" when they spot her (doesn't stop her from blowing up the base yet again). The crew also use the TIE Fighter stolen by Ezra and Zeb from "Fighter Flight" as a Trojan horse/escape vehicle during the mission to Mustafar.
Also in "Fire Across the Galaxy", Kanan and Ezra's final battle with the Inquisitor bears a remarkable resemblance to Obi-Wan and Qui-Gon's battle with Darth Maul at the end of The Phantom Menace. Two Jedi — master and padawan — fight a darksider wielding a dual-bladed lightsaber over a series of catwalks. One of the Jedi is knocked out, leaving the other to finish the job. The darksider dies (or appears to) when he falls into a power core. However, there are some notable differences as well. Instead of the padawan being the one to finish the duel, it is the master. In addition, the victor does not push his victim to the latter's death; rather, the victim chooses death, rather than face the wrath of his master.
During "The Siege of Lothal," Darth Vader states that he believes Ahsoka can lead the Empire to the other surviving Jedi who are in hiding across the galaxy. He specifically singles out Obi-Wan Kenobi before the Emperor tells him to remain patient. Anyone who has seen Revenge of the Sith knows that Vader is eager to get revenge on Obi-Wan after the latter chopped off three of his limbs and left him for dead on Mustafar.
In "Brothers of the Broken Horn", Hondo mentions that he was once good friends with a Jedi many years ago. Or at least he thinks they were friends. He's referring to his adventures with Obi-Wan Kenobi, which occurred in The Clone Wars.
In "Protector of Concord Dawn", Sabine reveals that she is (or was) a part of House Vizsla.
In "Homecoming", it's mentioned that Cham once fought alongside Mace Windu during the Clone Wars.
In "The Honorable Ones", Kallus mentions that his first unit was deployed to Onderon, and were subsequently ambushed by Saw Gerrera's rebels.
The population of Geonosis is mysteriously gone, according to "The Honorable Ones". Star Wars: Darth Vader reveals that it was sterilized to keep the construction of the Death Star a secret.
In "Twilight of the Apprentice", on Malachor, Ezra finds an ancient crossguard lightsaber (and a couple others have the same design as well) similar to the one used by Kylo Ren in The Force Awakens. This is a Call-Forward, as Kylo Ren based his lightsaber off of a design found from the remains of "the Scourge of Malachor".
In "Steps into Shadow", the Spectres are ordered to deliver the Y-wings they captured to General Dodonna's unit.
In "The Holocrons of Fate," Ezra sees a vision of what will eventually destroy the Sith. Kanan pulls him out of the vision before it goes too far, but Ezra mentions having seen "Twin suns..."
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 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.
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 New Teen Titans shorts do 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.
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.
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.
The Venture Bros.: Watching through seasons 1 and 2 again after seeing season 3, 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.
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.)
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.
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.