Sokka and Momo accidentally get strung out on peyote for a day and a night in the desert. Inevitably, Sokka was afterwards forever depicted by fans as the go-to character for any Mushroom Sambas.
Probably because as soon as he recovered, he licked something off a cave wall.
Katara: You've been hallucinating off cactus juice all day, and you eat something you found on a cave wall?
Sokka: I have a natural curiosity.
Aang's kiss screw-up in "The Ember Island Players". He kisses Katara once, she doesn't want it, and instantly a certain subset of fandom has decided he's an entitled "Nice Guy" who doesn't care about Katara as anything more than a prize and doesn't deserve to be the hero or have any girlfriend.
In the sequel series The Legend of Korra, Mako will forever be the guy who heartlessly strung Asami along, and then made out with Korra without bothering to break up with her (there actually was a breakup scene, but it was done very subtly so to many fans, that's how it looked).
Though it's helped by his portrayal in season 2. A big part of the problem was the show not properly acknowledging what a huge jerk he was to Asami, so when he does the same thing again, he ends up humbled and admitting he was wrong.
An arguable example is the Optimus Prime deaths. Each incarnation does die once or twice, and that's once or twice more than most characters who aren't from superhero comics, but you can't say "he's died a million times" unless you treat all Optimuses (Optimi?) as if they were the same guy.
Ultra Magnus is only remembered by fans for the infamous movie line, "I can't deal with that, now". Not only has it become an internet meme, but it also forever painted Magnus in fans' minds as an arrogant and lazy commander who would rather pass off responsibilities to his subordinates than actually get his own hands dirty. The fact that he's seen coordinating strategies more often than he is executing them doesn't help matters much, but he is more proactive than this misconception paints him as being. On top of that, the original line was said as he was in the middle of trying to pilot a ship that was crashing, so it was less "Who cares?" and more "Kinda busy here!"
Also, from the Transformers Movie, Ultra Magnus "usurps" leadership of the Autobots to the point of insisting the Matrix be merged with his spark. "Evidence" of this is shown by the fact that the Matrix only glows in the hands of Hotrod, but because Ultra Magnus is the de facto leader at the moment, everyone just ignores it.
Remember how G1 Red Alert was always panicky and ultra-paranoid? If so, that makes one of us: In the aptly-named episode "Auto Berserk", when a missile hits him in the face, resulting in brain damage that would have killed him eventually, he starts acting in this manner, to the point of helping Starscream get his hands on a superweapon just to keep the Autobots, whom he believes have turned against him, away. He's fixed later. Fans seem to forget both that he was this way for one episode only due to damage, and quite how dangerous the paranoid schizophrenic Red Alert really was. Even his toy bios go on about his paranoia, as if something we've never witnessed in any episode or comic but the brain-damage situation was in fact his defining trait.
A mild case of Older Than They Think: Toy bios were written (and often toys released) in advance of the characters turning up in related fiction and sometimes bore little resemblance to how the characters were portrayed in the cartoon or comic (Ratchet is described as a party animal, for example). Red Alert's paranoia was used for his Day in the Limelight episode but treated as a temporary state of affairs and ignored elsewhere.
Mix-Mix-Mixmaster's Verbal Tic of re-re-repeating the beginning of a sentence... some-some-something he only actually did in one ep-ep-episode ("City of Steel"). Thank-thank-thankfully. Shrapnel, however, really did repeat the last word of every sentence almost every time he appeared, appeared.
Beast Wars once had a damaged Waspinator think he was Shrapnel... only to start talking like Mixmaster did in "City of Steel".
Sentinel Prime may be going this way due to Transformers Animated, due to his smug attitude, general disrespect for others, and sometimes morally reprehensible techniques. In later continuities, there are some aspects of it, one of the names Zeta Prime has, suggested by the writer of Exodus is Sentinel Zeta Prime, and demonstrates much of the same behavior. The Sentinel in Dark of the Moon seems to avert this. But then it turns out he's worse than the Animated Sentinel, using both sides of the conflict and murdering Ironhide.
Hot Rod/Rodimus Prime probably would have been the Replacement Scrappy in any case, given how beloved Optimus was, but he really didn't help his case by being partially responsible for Optimus' death, distracting him at a critical moment. Oh, the fans who insist he did it deliberately...
Transformers even provides a meta-example: Transformers Animated came to a rather sudden end for various reasons. According to some fans, this means that Hasbro loves canceling things and produces shows, comics, and toys for the sole purpose of abruptly canceling them all. Any series people like is met with comments of "wonder if it'll get to episode/issue 10 before Hasbro pulls the plug on it?" and occasionally people ask whether a toy has been canceled after it's already out.
An in-Universe example occurs in Animated with Bulkhead continually bringing up the fact that Professor Sumdac was, well, deceived into rebuilding Megatron.
Tygra in ThunderCats is known as a weak-willed junkie, despite being addicted twice, only one of which was actually presented as a drug.
Aquaman is commonly haunted by his portrayal in the Superfriends cartoons; as a guy whose only real powers are swimming and talking to fish he was (almost) always useless, to the point of Memetic Mutation. This has actually worked out for him a little, as lateriterations of the character usually go to great (and awesome) lengths to subvert this image.
Wendy broke up with Stan exactly once, in the seventh season of South Park. They got back together in the eleventh season finale and have been together ever since. Yet fanfic writers tend to portray Wendy as a manipulative shrew who breaks Stan's heart again and again. This is usually done to justify Stan leaving her for Kyle. But even some writers who like Wendy latch onto the idea that she and Stan break up and make up all the time.
It may have more to do with the fact how shallow their relationship is shown at times. Both have shown heavy attractions to other characters in an extremely unsubtle manner (Stan once constantly attempted to make passes to a substitute teacher and even gloated about supposedly making love to her to Wendy's face while Wendy became disinterested in Stan in favor of another popular jock less than one season after they got back together). Their less than affectionate relationship was even Lampshaded in Raisins.
In Teenage Mutant Ninja Turtles fandom, April O'Neil's yellow jumpsuit, her fucking jumpsuit. Honestly, every conversation about the old show dissolves into one about her hotness and jumpsuit, this has even made it into other adaptations, as a reference or actually happening (Hell, in the Back to the Sewers season of the 03 show, she wears one!).
On Codename: Kids Next Door, Lizzie used a "Boyfriend Helmet" on Numbuh 1, and ordered him to destroy his teammates. In several future appearances, she was developed more, given her relationship more depth...but people's minds were already set. She became the single biggest target of Die for Our Ship in the fandom. Admittedly, she remained clingy, but nowhere close to that level.
Total Drama Island: Ezekiel made one sexist comment during his short time on the island. Two seasons later, even though others have shown far more sexist tendencies, Zeke was pretty much labeled Straw Misogynist, and is still hounded for it.
Similar to the Pikachu/ketchup example, Starfire of Teen Titans was shown drinking mustard once. It's common for it to be referenced in fan works.
She is, however, shown at several points later with a bottle of mustard with a straw in it in the background, even if she's not actually in the process of drinking it.
Spider-Man: The Animated Series - whole show, for the censorship issue. Yes, they couldn't say "die" or "kill," have Morbius suck blood, or have Spidey full-on punch anyone. But it seems that's all anyone says about it lately. In fact, it was an ambitious arc-based series with a lot of depth, emotion, surprises, and the portrayal of Venom that has impacted every adaptation since. Interviews with showrunner John Semper reveal that as it was on Fox - yeah, thatFox - it was a 24/7 battle to keep it that way. And just because they couldn't say "die" doesn't mean no one ever did die.
Professor Farnsworth from Futurama, having been declared dead as a tax dodge some years before, found himself irritated that his son Cubert used this fact to perform a hostile takeover of Galaxy Express.
Farnsworth: You take ONE nap in a ditch and people start declaring you this and that!
And of course, Leela's infamous one night stand with Zapp Brannigan is mentioned every time Zapp makes an appearance in the show. Usually by Bender or, more commonly, by Zapp himself.
Sym-Bionic Titan: Xeexi got his Memetic Molester status by mind raping two characters in an episode. Only one instance is really ever noted, but now Xeexi is only remembered as a rapist, even though he's also a tactical genius, and well... dead.
Derpy Hooves is all about this trope. Her first prominent appearance in The Last Roundup has her voice changed and removed her being called by name, on the basis of her being considered offensive; absolutely nobody involved — including several people and ponies not involved — have been allowed to live it down.
Fluttershy wanting to be a tree, based on a random night conversation on a train. This received a Call Back in "Hurricane Fluttershy" where she hides inside a tree costume to avoid participating in the local pegasi's efforts to supply Cloudsdale with water.
Her Sanity Slippage in "The Best Night Ever" forever convinced a large section of fans that she's secretly a Yandere.
A Meme in which Scootaloo is secretly a chicken, based on a short bout of teasing from her friend Apple Bloom.
Pinkie Pie being a sadistic serial killer who may or may not have severe childhood trauma, based on her Sanity Slippage in "Party of One", as well as a certain infamous fanfic. Never mind that in the episode she was, at most, bipolar and manic depressive, which are many times more likely to result in self harm than any violent tendencies.
Celestia banished somepony to the moon once (a Mad Goddess whose scheme would have killed all life in Equestria), and even then only with the help of the Elements of Harmony. This doesn't stop numerous fans from depicting her as solving all her problems by banishing them to the moon, and even having her own Catch Phrase ("TO THE MOOOOOOOOOOOON! BEEYETCH!"). The wildly popular fanmade short Friendship Is Magic Bitch does not help matters (in fact that's where said catch phrase originates from).
Before that, she (and Luna, which no-one seems to remember) turned Discord to stone... after he conquered Equestria and turned it into a nightmarish World Gone Mad. Naturally this led to depictions of "turn it to stone" being another one of her habits along with "send it to the moon".
Funnily enough, Discord seems to think this In-Universe. "Unlike you I don't turn ponies to stone!"
Rainbow Dash said "twenty percent cooler" one time, and "ten seconds flat" two times. To a lot of fans they might as well be her Catch Phrases.
A subversion with this one, actually. Not only did Twilight have some basis for Sanity Slippage in "Swarm of the Century", but she did it again in "It's About Time". Furthermore, she is slowly learning to control it as of "Games Ponies Play". The fans have spoken about her mental health, and the writers are listening.
At the start of Season 3, Spike tells her that a test is "just a test". Twilight's reaction almost blows up the library.
Detractors against Twilight becoming an Alicorn Princess will almost always point this out in spite of her Character Development.
Derpy was interested in having some muffins once when they were served to a group of ponies, and everyone went with it. Her toy's box even features muffins.
Made even more egregious since nobody can actually be sure whether muffins were even Derpy's suggestion - the word "muffins" matches the lipsynching of at least one, possibly two other ponies.
People tend to believe that Celestia and Luna are useless princesses, the former being not as powerful as she claims to be, while the latter is apparently lazy and arriving long after the battle is over. These only happened once in one episode, and the former was moderately justified, yet people tend to make it out like they do this all the time.
Shining Armor did a Fastball Special with his wife Cadence exactly once, in a life-or-death situation where there was basically no other option. Since then the fandom only knows him as "Wife Thrower" (albeit affectionately).
Lyra Heartstrings has once been seen sitting on her haunches (like a human would) on a park bench in episode "Dragonshy". Since then, you could not going anywhere without someone posting up fan fiction, music videos, comics, and the like, depicting Lyra with some level of human-obsessed fanaticism, either interested in proving humanity's existence, gaining hands, or becoming human herself, ignoring everything else.
Sweetie Belle gained somewhat of a reputation as The Ditz because she didn't understand "more than I can chew" was a figure of speech.
The Canterlot Wedding has moments that most people will never live down, ranging from Twilight accusing Cadance of being evil despite having no evidence (Though she was right, just not in the same context she was thinking), to Shining Armor yelling at his sister and more or less cutting off his ties with her and never apologizing for it once Chrysalis got kicked out of Canterlot, to Twilight's friends abandoning her for a pony they just met a day or so ago and not even giving Twilight a chance to explain her doubts before the big accusation scene, to Celestia and Luna being easily beaten and absent respectively, to even Chrysalis having made mistake after mistake, even when everything is going according to her plans. Needless to say, the season two finale has one of the largest broken bases before Season 3 rolled around.
The episode "Feeling Pinkie Keen," in which writer Dave Polski worded the moral so badly that it ended up sounding more like "Science Is Wrong and you should just accept that things work the way they do without trying to understand it." And a bit of pushing religion on kids in there, too.
In X-Men: Evolution, Scott was always a nice guy to everyone unless they deserved otherwise. However, he and Avalanche had a known rivalry due to leading opposing teams, usually Avalanche instigated it, and mostly Scott was defending someone else or simply reacting. However, during season 2, Avalanche had changed. Not as in he developed, but his personality had completely taken a U-turn as he was suddenly in love with Kitty and willing to join the X-Men simply to be close. Obviously, Scott remembers how he's acted and doesn't trust him. When Avalanche proves he's not interested in learning at the institute Scott retaliates by reactivating the Danger Room control for a second to make him trip up, a complete Out-of-Character Moment (he made a joke, completely forgetting whothis is). And when someone starts stealing the X-Vehicles, Scott, as well as EVERYONE thought it was Avalanche. When its all over, Scott apologized for this and offered to start again. Lance decides to quit because, after his time here, he's realized its too much work and not really him. However, the Brotherhood fans and a few others refuse to think of Scott as anything but a petty jerk who, without reason, just hates Avalanche who is nothing but a saintly loving Boyfriend to Kitty (despite that by the end of this season he returns to previous characterization and breaks up with Kitty), and that Scott was the reason he quit, having driven him away with his bullying and accusations when Avalanche would have made a great addition to the team. Sigh. That's two characters remembered for a personality they didn't have, because of one episode and one Out-of-Character Moment.
Similarly, the New Recruits are only remembered by some fans as idiots who only want to cause trouble and if left alone would destroy the world. They only did two stupid things, steal the X-Vehicles, and break into the Danger Room unsupervised. Not only did they learn their lesson, they actually showed they were smarter than previously thought (how many of you could fly a jet or survive a Danger Room session designed for the more experienced children?), in fact, it wouldn't have been a problem had Avalanche and Kitty not got in the way and caused the systems to get fried and fired at two Air force jets. But because one caused Avalanche to quit and another was 'confirmation' people always think of them as Too Dumb to Live assholes.
Many characters in The Simpsons are constantly lampshaded by the things they did in the past (especially the secondary characters). Some character only become known by the things that happened to them.
Meanwhile, more prominent characters in the series have past status quo changes lampshaded around them. For instance, there is Apu, who is now currently defined by his marriage to Manjula, his eight children, and his affair.
As for Lisa, occasional references have been made towards her vegetarianism and Buddhism, both of which were made permanent into order to get celebrities Paul & Linda McCartney (3F03) and Richard Gere (DABF02) to make their one-episode appearances in the respective episodes.
Then there's Seymour Skinner, who has been referenced by his false identity, as well as his relationship and break-up with Edna Krabappel.
The town of Springfield as a whole will probably be never forgiven for driving Bart to kill himself in "The Boys of Bummer".
In-universe example: In a Robot Chicken skit on G.I. Joe, a new recruit is given the name Fumbles after slipping on his spilled soda. This leads him to defect to COBRA for the sake of revenge. Unfortunately for the Joes, "Fumbles" happens to be a Badass BookwormCold Sniper who is capable of single-handedly killing them all.
Even better, at the end of the sketch, Cobra Commander is so impressed that he wants to give the sniper a much better nickname, only for him to coldly respond "It's Fumbles. It's always been Fumbles."
Partly due to Seinfeld Is Unfunny, Dreamworks Animation is still often known for "not being Pixar" or "pop culture references". This is despite the fact that their films have gradually moved away from this (heck, some of their earlier films just avoided it).
Disney is still constantly mocked for all the direct-to-video sequels they churned out, even though they stopped making them after John Lasseter took over.
While it's understandable since "When you wish upon a star" is basically the theme of the Disney company, Disney Animated Films are often generalized into only promoting that moral (that a princess, or any person, just needs to want something and they'll get it). The Princess and the Frog seems to address this prevalent idea, and balanced it out with a hard work moral.
The same applies to the idea that all classic Disney cartoons are generally bland and cutesy stuff. Yes, there are a lot of cartoons in the Disney canon that fit that trope, but Uncle Walt and his studio also made a lot of impressive technical innovations in the animation business. Look at all the details and realistic atmosphere in films like "Snow White", "Pinocchio", "Fantasia" and "Bambi" and compare that to the art of most other animation studios!
Also, a lot of the early Disney cartoons from the 1930s and 1940s have scenes that are pure Nightmare Fuel, almost on the level of a horror movie for adults!
It's been two years and they're still complaining about Winnie-the-Pooh being released the same day as the final Harry Potter film, calling it a sign of Disney's declining faith in 2D animation, though it did moderately well in the box office (making up its $30 million budget) and Disney said it lived to its expectations.
Most of the classic cartoons from the 1930s until the 1950s have a reputation for being nothing more than violent slapstick with a lot of explosions and people hitting each other. Despite the fact that the Bugs Bunny and Daffy Duck cartoons, for instance, are very dialogue driven and a lot of comedy also comes out of the dialogues between the characters.
For that matter, the Censored Eleven are disproportionately referenced in media, even though they comprise less than 1% of all Bugs Bunny cartoons.
Superboy in Young Justice has yet to lose his Emo Teen label, despite the fact that he grew out of it after the first ten episodes and lost it completely in Season 2.
No matter what Brian from Family Guy does that might redeem himself, certain fans will never forgive him for vocally espousing his ultra-liberal viewpoints, particularly in the episode "Not All Dogs Go to Heaven."
In Ed, Edd n Eddy, despite showing some Character Development, albeit smug and a follower, Jimmy will always be known for framing the Eds in the episode "If It Smells Like An Ed."
Similar to the Mixmaster example above, Roadblock from G.I. Joe is known (and frequently mocked) for his Rhymes on a Dime dialogue; while he did talk like that, he didn't do it all the time. However, most parodies of the character make him say every sentence in rhyme, as if he always did.
One season 2 episode of Superjail! depicted the Twins as cannibalistic, insect-like, and also consuming bleach and other toxins. Even though some of the alien traits depicted in it were not referred to again in the following season, the episode definitely left a mark on fans' minds and fanart and comments will mention them in some way. For some, the episode seemed to ruin the characters and exaggerate the alien nature a little too much. And then you have the fans who claim the characters should never have any fans that like them over the actions they pulled in the episode.
Alice, er, "assisting" the Mistress in the season 3 premiere goes down this way for other fans. Never mind whatever else Alice did before or after that episode, there are some that give her the reputation of "Ruined Warden/Mistress foreverand is in the way".
Hey Arnold! an in-universe example, one episode had Rhonda throw a party in which some characters were not invited for being Geeks. They bring up the literal definition of the word, which repeatedly prompts this exchange: "...And none of us bite the heads off chickens! Except Curly." "Yeah! And that was only the one time!"
This is best illustrated in Itchy and Scratchy where the two main characters are blatant Expys of Jerry and Tom respectively. The show's portrayal of the Jerry-like mouse character is that of a sadist with a penchant for murdering not only Scratchy but any other cat that happens to appear in the episode. The opening sequence shows that the violent roughhousing is meant to be an equal rivalry, but there have been few if any episodes (all of which escape memory) to support this.
Dana Tan, Terry's girlfriend in Batman Beyond, is often called a Damsel in Distress who's constantly getting rescued by Terry/Batman. In fact, she only has to get rescued in one episode and is briefly in danger in the movie, and neither time does she passively wait for rescue.
In-story, Terry is constantly reminded of his delinquent record, despite having spent months in juvenile hall.