In a season two episode, Ozai and Iroh's father Azulon is said to have been Fire Lord for 23 years before Ozai and that Fire Lord Sozin ruled for the first 70 or so years of the 100 year war. The Nick website also provided a brief backstory for Sozin saying that he was a young man when he became Fire Lord shortly before Roku died and was prevented from starting the war until the elder Avatar Roku's death. Come season three where it's revealed that Sozin and Roku are actually the same age and that he was an old man when he started the war. In response, the creators revealed that Sozin ruled for only the first 20 years of the war before Azulon took over for the next 75 years before Ozai became the Fire Lord for the last 5 years. This has caused confusion for many fans.
Katara mentioned early on that the Fire navy shipwreck near her village has been around since her grandmother was a little girl. Fast forward to the end of the season where we learn that Katara's grandma isn't even from the Southern Water Tribe and didn't arrive there until she was no younger than sixteen.
Early on it's implied that Avatar Roku lived for more than 100 years and that Kyoshi has been dead for multiple centuries. We later learn that Roku was actually only about 70 when he died and Kyoshi died (though still a long time ago) less than 200 years prior to the present at age 230.
Furthermore, in the third episode Avatar Kyoshi was not shown as the Avatar before Roku, in her place there was a male Earth Avatar. This was quickly fixed in the next episode.
On the show's website it suggests that Kyoshi was born on the island later named after her, yet we later find out that it wasn't even an island until she made it one.
"The Why Of Fry". This fourth-season episode posits that the original accident causing Fry to be frozen and sent forward into the year 3000 was actually intentionally caused by Nibbler and the Nibblonians so that he could save the universe from the giant space brains. At first glance, this seems like a retcon; however, careful viewing of the original first episode shows the shadow of Nibbler underneath the desk as the accident happens and in a revisited scene his eye stalk poking out, proving that the producers of the show set this up from the very beginning.
In Lethal Inspection, we see the real story of Bender's birth after he told a different version. Rather than being born full-sized just a few years ago, he was born as a baby-shaped robot with a visibly-younger Hermes as his inspector. Completely worth it for that song.
Bender caused the alien apocalypse we see in the first episode.
One of the biggest retcons was the episode "Where No Series Has Gone Before", where it is revealed that it is illegal to even mention Star Trek. Virtually every episode before that had at least one Star Trek reference. When Fry meets Leonard Nimoy at the Head Museum he calls him "Spock". When Fry is confused by the DOOP, Hermes says it's like the Federation on Star Trek. Also, in a DVD commentary, one of the writers suggested that characters were saying "Start Wreck" rather than Star Trek.
Kim Possible: While it was never mentioned in the show proper, the Word of God in the beginning was that Shego's plasma hands were due to some technology in her gloves, which matched with how her powers were used in first season episodes. She was eventually turned superhuman, shown using her abilities barehanded, and even given a Super Hero Originbackstory. Note that the official website still has the original explanation.
The Justice League cartoons get rid of most of the annoying retcons in The DCU, but they make a few new ones. Doomsday showed up in the original Justice League episode "A Better World," in which he said he was an alien invader looking to see what Earth had to offer in the way of worthy opponents. He's never heard of Superman before and doesn't seem to care one way or another about him. Later on in Justice League Unlimited he was retconned to be a creation of Project Cadmus, who reverse-engineered him from Superman's DNA and conditioned him to hate Superman above all else. Then, to explain his appearance in the older episode, Cadmus shot him into space. His voice even had modulation added to it.
Batman Beyond threw together plot points from dozens of episodes and two movies into something they clearly weren't originally meant to be. It's cleverly pulled off, though, as any potentially created inconsistencies are rather smoothly Hand Waved by a line from the creators, stating that their choice to have Bruce Wayne revealed as Terry and Matt's biological father was partly motivated by them realizing how Terry and Matt having black hair is genetically improbable since Warren's hair was light-brown and Mary is a redhead.
The series has gone through a retcon as of the episode "That 90's Show." In the earlier episodes, Homer and Marge attended high school in the late 1970's got married three or four years after graduating from high school after Marge became pregnant with Bart, Bart was born in 1980, Lisa was born in 1982, and Maggie was born in 1989, but the series was retconned in the aforementioned episode. Marge in previous episodes had never gone to college, but was suddenly attending in the mid 90's; it led us to believe Homer had a Grunge band, whereas in an earlier episode he didn't understand grunge at all; and they also showed us that they dated for at least ten years before getting married and having the kids, who are now retconned into being born in the early 2000's. "That 90's Show" itself seems to be retconned since, though with the floating timeline it means Homer and Marge should be older than their 30s.
Prior to this the characters were stuck in the 90's and never aged.
The Simpsons time line being what it is, this retcon actually makes sense. The Simpsons' time exists on a sliding line, so even though every episode happens in "the present", the past keeps moving forward. Also note that while the kids' ages never change, Marge and Homer have been, albeit very slowly, getting older; being in their mid-thirties in early episodes and now nearing forty (presumably to reflect the aging American population), which does leave a gap for the events of "that 90's show" to take place.
The writers have made clear that some things are immune to the sliding time gimmick. Grandpa Simpson and Mr. Burns will always be WW2 veterans, even if that makes them unrealistically old. They will also have both known the Great Depression.
Also on the subject of Mr. Burns, he started out at 81 years old (the early episode where Homer defrauds the plant's health insurance for hair growth), but is now canonically 104 years old.
Mr. Burns had been mentioned to be 104 as early as season 6. Although, his age is basically debated, many of his other mentioned possible ages place him as being older than 104, some place him at 118, others at 122.
Mr. Burns' age has also been said to be four digits inThem Robot, and even more jarringly his place of birth has been stated to be Pangaea, a super-continent which broke apart in the Triassic period.
Unrelated to the sliding timeline, one glaring aspect that was essentially retconned from the show is Homer's relationship wtih his mother. In the fourth episode, "There's No Disgrace Like Home", Homer mentions that his mother said he was a big disappointment. However, if "Mother Simpson", "My Mother the Carjacker", and "Mona Leaves-a" are any indications, Mona Simpson has always loved Homer and it's doubtful she would've ever said anything of the sort.
As for the whole 90's ret-con, it helps to clear up one glaring Plot Hole that Jon Stewart once pointed out to Matt Groening in an interview. Homer and Marge were supposed to be in their late 30s. They got married when they found out Marge was pregnant with Bart, but since Bart is only 10, about a decade would had to have passed between high school and Bart being conceived. Lisa even lampshaded said plot hole in the episode.
A season 3 episode (Bart the Murderer) establishes Fat Tony's name as William "Fat Tony" Williams. Several seasons later, he's canonically named Anthony "Fat Tony" D'Amico. It's either a case of this or the writers forgetting they already named him.
In "Death Picks Cotton", when Cotton tracks down Hank and company in a Japanese restaurant, it triggers his flashback, and he calls the Chef (who only speaks Spanish) a Tojo. However, part II of "Returning Japanese" had basically been about him forgiving the Japanese, and he was talked out of his scheme to spit in the Emperor of Japan's face by his illegitimate son Junichiro, resulting in apparent Aesop Amnesia. For that matter, that episode Flanderizes him back into the Jerk Ass, verbally Abusive Parent he was at the beginning of the series, throwing out his Character Development into a Jerk with a Heart of Gold.
Peggy's background was rewritten so that she spent most of her early life in Montana, rather than spending her high school years in Arlen. This complicates the episodes where Hank and Peggy are shown as High School Sweethearts. Meanwhile, Peggy's mother changed from an older-looking version of Peggy who was just hyper-critical into a totally different looking woman who was a downright bitch.
Luanne's father/Peggy's brother didn't appear until near the end of the series, though was mentioned several times. Originally, he had fled to an oil rig in fear of his abusive ex-wife, refusing to come back on land until Hank faxed him her death certificate. Bill mentions him looking like a male version of Peggy while Hank mentions getting along with him. When he shows up, however, he is revealed to have actually been in jail for years for being a con artist and thief, a fact which Peggy hid from everyone with the oil rig story. He also looks nothing like Peggy and it seems that Hank had never met him before. This change calls forth several bits of Fridge Logic: one, Luanne is supposed to have seen the event that caused him to flee to the oil rig; how does his incarceration fit into that event, especially since we know her mom was put in jail herself for that abuse? And if she was already in jail, why did Peggy (and later Hank) think it would be so traumatizing for Luanne to also know that her father was as well? Also, they established several times that Luanne witnessed the fight between her parents that led to her mom stabbing her dad with the fork which occured in the very first episode where she was about 16 or 17, but in this episode her dad claims the last time he seen her she was a little girl about five years old and she has no memory of the event.
Hank and his old Arlen High School football team challenged the team that they lost against during the championships to a rematch that they eventually win. This one is made more annoying by the fact that Hank had come to terms with losing the game in an earlier episode. It left a bad taste considering it was one of the last episodes.
Another would be the origin of Dale's Rusty Shackleford identity, in a couple of early episodes he mentions he got the name from a boy who died from smallpox back in the 1950's, however it was later retconned in the season 11 episode "Peggy's Gone To Pots" where he supposedly got it from a boy who went to his school and moved away and Dale thought he died, and the "real" Rusty Shackleford arrives to tell him to stop using his name.
Thomas the Tank Engine: Reverend Awdry stated that the North Western Railway was built in 1914, mainly by Edward. In the 2009 movie Hero of The Rails, it is stated that the new characterHiro was responsible for building it.
In the fifth episode we are introduced to Dexter's rival, Mandark, who had just moved into the neighborhood as an exchange student. He introduces himself as Astrononminov (possibly his last name, but prefers to be called Mandark). He was a fairly competent villain, at least when Dee Dee wasn't around, and he had a sister named Olga Astrononminov that prefers to be called "Lalavava". In the last two seasons (made after a production gap of a couple years), Mandark is now named Susan and has a pair of hippie parents. He and Dexter supposedly first met when they were little, and he became an incompetent villain because Dexter made fun of his name.
Olga seems to have been forgotten by the writers entirely aside from her one appearance. Season 3 and 4 episodes went out of their way to exclude her.
In one Oh Yeah! Cartoons episode Vicky mentions having a little brother. This brother was never heard from again but instead she has a little sister named Tootie.
In the original pilot, Vicky was apparently babysitting Timmy for the first time, and Timmy was ten. The first movie retconned both points: Vicky had been Timmy's babysitter for a year, and Timmy was nine when he first got Cosmo and Wanda.
The season nine episode Let Sleeper Dogs Lie retcons Denzel Crocker's childhood as shown in season three's The Secret Origin of Denzel Crocker! The episode shows him to have always been complete jerk, and that he lost his fairies due to turning 11, rather than being a nice kid who only lost Cosmo and Wanda due to Timmy interfering with his life and causing him to reveal the existence of his fairies, thus turning him bitter. This also retcons the idea previously established in Channel Chasers, where beyond revealing the existence of your fairies, the only reason you would lose them is if you either became an adult (which was Timmy's case) or your life has improved to the point where you no longer need them.
In the made for TV movie Jonny's Golden Quest, Jessie was revealed to be the daughter of Race Bannon and his mysterious lover Jezebel Jade, during a brief affair the two had. In the second season of the next series The Real Adventures of Jonny Quest, the new writing staff created the character of archeologist Estelle Vasquez to be Jessie's mom (and Race ex-wife) since they believed Jade wasn't the kind of woman who would settle down.
Jesse Bannon herself was a retconned version of Jesse Bradshaw, a character who appeared in an episode from the 80's series The New Adventures of Jonny Quest. The original Jesse was not related to Race Bannon.
In the Ben 10: Alien Force episode "Be-Knighted" the evil group the Forever Knights has their Villain Decay cemented when it's revealed that their ultimate goal was no more than to slay some poor alien dragon they had captured as opposed to conquering the world, as they had attempted in the original series (they even had a different leader than was seen in the older series as well) because they're knights and, well, slaying dragons is what knights do. When Ben and company helped that dragon escape, they decided to eventually travel to that dragon's planet to try and wipe them out. Series writer/producer Dwayne McDuffie later revealed this was, in fact, their goal from the beginning and the Forever Knights Ben fought in the original series (led by former Plumber Driscoll as the "Forever King"), was actually a rogue splinter faction of the original group and that the king Patrick, seen in "Be-Knighted", was the true Forever King and Driscoll was an imposter, or something. Ben 10: Ultimate Alien changes it again. There are in fact many factions. Driscoll's was one, the ones literally Bullying a Dragon were another, but all were considered as having lost their way by the true founder, who returns and brings them all together (in an episode in which we see every Forever Knight leader from the past, all of whom are willing to follow the First Knight once he proves he's who he says he is). Their original goal was to get rid of an Eldritch Abomination called Diagon, but they seek to remove all"alien scum" from the Earth. This results in a Darker and Edgier arc beginning, where Diagon is the ultimate evil in the series and the Forever Knights are a very competent foe (and help in episodes related to their common enemy).
There's also changing the nature of Gwen's magic due to being part alien and turning the Plumbers from a defunct federal Men In Black-esque agency into a still-active intergalactic police force.
Kevin has his past added onto to make the complete overhaul of his powers and personality between the original series and AF/UA make at least more sense than it did originally. Also part alien; has the earlier energy powers and the new matter powers, but using energy makes him go nuts, making for the Kevin of the original series. Kevin was retconned further in Ben 10: Omniverse, where it was revealed that he was not part alien at all, but the Ben 10-verse equivalent of a mutant.
The Season 14 episode "201" retcons the twist ending of "Cartman's Mom Is Still a Dirty Slut", from way back at the beginning of Season 2 (twelve seasons before) by revealing that Liane was Cartman's mother all along and her claim of being a hermaphrodite was simply a cover-up to protect the identity of Cartman's real father, a former right tackle for the Denver Broncos. Said Denver Bronco turns out to be none other than Jack Tenorman, the man Cartman murdered and fed to his son Scott in the Season 5 episode "Scott Tenorman Must Die". Cartman seems more upset at the fact that he is Scott Tenorman's half-brother, thus making him half-ginger, than with the revelation that he killed his own father and then fed his remains to Scott. He feels better when Mitch Connor reminds him of the fact that he's also half-Denver Bronco.
South Park also gives us what is possibly the most epic retcon ever. In the second episode of the "Coon and Friends" trilogy, Kenny reveals that he can't die. He explains to another superhero that he's died many times in the past, and just ends up waking up in his bed like nothing happened, the only one who remembers his gruesome demise.
In the episode "City Sushi", Tuong Lu Kim the Chinese restaurant owner is revealed to be another personality of Dr. Janus, a doctor who suffers from multiple personality disorder. In his past appearances, he was portrayed as an authentic Chinese man.
In "Mr. Hankey The Christmas Poo", it's established that Kyle's family are the only Jews in South Park. Later in "The Passion Of The Jew", we see a Jewish community large enough to have a synagogue (though the synagogue first appeared in "Cartmanland").
Lampshaded in the 9th season episode "Excellence in Broadcasting": When the Griffins hear that Rush Limbaugh is coming to town to promote his latest book, Chris recalls that, during Lois' brief stint at Fox News (in the "Foxy Lady" episode), she reported that both Limbaugh and Michael Moore were characters played by Fred Savage. Lois dismisses this, explaining that even if something is true to begin with it becomes a lie when said on Fox News.
The episode "Brian: Portrait of a Dog" shows Peter adopted Brian as a stray when he was washing people's cars to earn money, however "The Man With Two Brians" shows the Griffins got him as a puppy.
Joe's first appearance in "A Hero Sits Next Door" has the cause of his paralysis be falling from a rooftop after a fight with The Grinch. In season 11, it's revealed this story was something Joe made up and he's actually crippled from being shot.
In the fourth episode, it is stated that Roger has been living with the Smiths for four years since Stan rescued him from the C.I.A., however in a much later episode there's a flashback of Stan giving him to Steve as a little boy for his 10th birthday.
In a flashback from "White Rice", it was shown that Hayley once has a twin sister named Bailey who is implied to have died because Stan didn't allow them to get vaccinated. This goes against an earlier episode that showed Hayley as being a single birth (though that could have been a throwaway gag to make Stan look bad).
Played for Laughs in "Stan's Best Friend", wherein Stan refuses to let Steve get a puppy:
Played straight with Francine's adopted sister Gwen. In the early episode "Big Trouble in Little Langley" she was established (via dialogue) as being a Brainless Beauty who needed all the help she could get to survive, which was why her parents were leaving everything to Gwen in their will, trusting their 'smart daughter' Francine to be okay without their money. Years later in "Now and Gwen", when Gwen finally shows up on screen she does turn out to be beautiful, but of average or better intelligence and a hardened criminal. Dialogue prior to "Now and Gwen" also said that she was younger than Francine by three years until the aforementioned episode made her the oldest by the same number of years.
Lampshaded in Frisky Dingo. In one episode, Grace Ryan is seen taking "Ret-Con" brand ant poison to cure herself of her Superpowered Evil Side Antagone. In another episode, it's explained that the Annihilatrix has been completely rebuilt, despite previously having been stripped down for scrap. A quick cut shows that the rebuilding was done by "Ret-Con" Construction Company.
In the Rocky and Bullwinkle story "Missouri Mish Mash", this occurs within the same story. At first it's said that the Kirwood Derby has been around since the Stone Age. Later on however, it's said that the derby was created for an otherwise idiotic moon prince.
The Arthur episode "Arthur and the True Francine" showed that Muffy first came to Elwood City when the others were in second grade. However, the first season episodes are filled with Early Installment Weirdness which was later ignored. Brain's Shocking Secret showed that Muffy has been in Elwood City since kindergarten.
When Morph is revealed to be alive in season 2 of X-Men, the flashbacks of his survival don't match the events of "Night of the Sentinels". In the original pilot, Jean telepathically senses Morph getting shot by the Sentinels and, only seconds later, Xavier tries to reach him, only to find no trace of his mind, implying he's dead. Meanwhile, it's shown that the X-Men were under siege by the Sentinels for at least several minutes before deciding they couldn't help him (or an also injured Beast) and reluctantly retreating. In the season 2 flashbacks however, Morph is shown to be still alive until after the X-Men leave. It's even worse in the X-Men Adventures spin-off comic. The second issue gives Morph an on screen death (something the cartoon never did), with Beast by his side, and even subtly explains what happened to his body (it was seemingly never found in the show). When season two rolls around, they just flashback to Sinister carrying him to safety, without ever explaining the inconsistencies with issue two.
Fillmore! episode 3 has Ingrid learn about Fillmore's delinquent past. Episode 8 establishes in flashback that he told her about it before even recruiting her to the Safety Patrol. It's unlikely Ingrid forgot, given that she literally has an eidetic memory.
My Little Pony: Friendship Is Magic has a noticeable case of Remember the New Guy. In Pinkie Pie's backstory there's no sign of a fourth Pie child but a later episode shows a picture of the day where she has a random new sister and Maude appears later as Pinkie's favorite sibling.
A Pup Named Scooby-Doo has a jarring one involving Daphne's parents. In the episode involving Daphne's room being stolen, her parents are shown to look fairly different from her (in a sense that they don't look exactly like her) with an example of her mother having blonde hair. In the second-to-last episode of the cartoon, Daphne's parents are seen again, except this time they're adult carbon copies of her appearance wise and even have her "There's no such things as ghosts!'' verbal tic.
The title character of Sofia the First was hyped up as Disney's "first latina princess" however there was a backlash because she is a fair-skinned, brunette girl. A few years later Disney seems to have gone back on this statement as Elena of Avalor is being referred to as their first latina princess while Sofia seems to be from a Spain inspired country.