Priss Asagiri falls victim to this in the final episodes of Bubblegum Crisis Tokyo 2040. She reveals to her love interest, ADP Officer Leon McNichol, that the real reason she hated the AD Police was that they broke her favorite CD during a boomer attack. Before then, she was developed as a complex, mysterious, stoic loner. This revelation derails her integrity for the sake of cheap laughs and misguided social commentary.
Daichi Misawa of Yu-Gi-Oh! GX, whose Teen Genius personality became grotesquely twisted out of shape until he began acting directly contrary to what had previously been shown. (Then again, a drastic personality change is justified if he really was raped by a love-crazed Amazon.) He eventually snaps/has an epiphany and runs naked through the school grounds a la Archimedes, and then gets Put on a Bus. He eventually returns but does little before deciding to live in another dimension with said love-crazed Amazon. This was arguably a minor attempt to return his character to its roots as he made this decision partly because he felt more appreciated in the other dimension than he did in his native one and acceptance/recognition was what he had sought from the start.
For a good number of people this happened in Season 4 of the anime, to Yami Yugi. He freaks out when nearly losing a duel despite having far developed out of this trait as per the manga, and is goaded by his opponent into playing a card that would make him go against his (or rather, Yugi's) value system. Unfortunately however, this is heavily a result of 4Kid's dub-based changes in characterization compounding a recurring error about Yami Yugi taking things too far when pushed, which the series HAD been trying to call out at the time as it hadn't been addressed since Yami Yugi almost killed Kaiba in Duelist Kingdom and Creator Kazuki Takahashi admitted himself had forgotten to deal with, but had left to the season to do so.
Happens to Kaiba throughout the series to a lesser extent both canonically and once more due to dub-based exaggerations of character, as he continually forgot lessons about teamwork...though that is more caused by the series' original writer continuously railroading Kaiba into the role of antagonist rival to Yami Yugi even past the point where they'd settled their issues.
For quite a few people, the change in Aki's character was too abrupt, most of it within the space of one or two episodes rather than being more evenly spread out. This was not made better by the Dub skipping episodes and playing them out-of-order which resulted in this feeling.
Unfortunately, this is a result of real-world events screwing the show over which resulted in the show's mid-production direction-change that plagued the entire third story arc. A cult conspicuously similar to the psychic cult in the series performed a terrorist attack during the period 5D's was on the air, forcing most of the storyline the show had planned to run with entirely thrown out, leading to a scramble for a new direction that wasn't found until the tie-in crossover film and fourth arc of the series started. Worse, Carly's voice actress was a member of said cult, making the producers forcibly demote the role and their romance until they had sufficient cause to fire her and recast the role...around episode 140 when the show was wrapping up.
Yugioh VRAINS: GO Onizuka goes from being Playmaker's hot-blooded, prideful, but good-natured rival who loves to entertain the crowds in the first season to the second making him a power-hungry jerkass who's single-mindedly obsessed with gaining strength and avenging his losses at Playmaker's hands. Explicable due to Vrains not having a consistent writing staff.
In Transformers: Generation 1, Cyclonus and Scourge were a pair of dangerous, cunning, devious warriors. In Transformers: ★Headmasters, they suddenly became a pair of bumbling idiots who kept getting in Sixshot's way, and wondered why Soundblaster lost all respect for them. They were quietly dropped from the show soon after.
Rodimus Prime after the movie also suffered derailment, turning from 'plucky youth' into 'useless jerk' over the third and fourth seasons.
My-Otome: Nina Wang was introduced and developed as a somewhat Friendly Rival to Arika and a more-than-competent fighter, celebrated as one of the top students at Garderobe Academy. Following the mid-series Wham Episode, that rivalry collapsed as she underwent a FaceHeel Turn and became a puppet of the Big Bad desperate to prove her loyalty to/love for her adoptive father Sergey at the expense of everything else.
This occurred in Tytania, and it happened to, of all people, Estrades- the man turns from a calm, devious schemer who was perfectly fine being ordered about by his brother Ajman to a wild, rabid, hateful person who despised his sibling with every fiber of his being. To top it off, his derailment is combined with a severe case of a mixture of the Idiot Ball and Too Dumb to Live when he offs himself by shooting himself after tumbling down a flight of stairs.
Dumas from Kiba starts off as the strong, quiet champion of Tempura/Templer, then makes a shocking FaceHeel Turn and would have lead Jimoto's invaders to overtake Tempura had not Zed's Spirit gone on a rampage and stopped them. The really interesting part was that Duma's reasons for this were pretty reasonable: because of his family history, he basically wanted to shape his own destiny. However, after Zeds Spirit rejected him, which took a toll on his sanity, he became a regular psychotic baddie whose sole purpose in life was betraying Tempura in every way possible and becoming royalty.
Ash's Charmander, upon evolution into Charmeleon, goes from arguably the nicest of Ash's Kanto starters to volatile and uncontrollable. Upon evolution into Charizard, it becomes a complete Jerkass that rarely even tolerates Ash until halfway through the Orange Islands arc when Ash saves its life (again). While Charizard's behavior was intended to teach Ash some humility, it still comes completely out of nowhere and is treated entirely as Ash's fault, even though hehadtried controlling it before and Charizard actively refused to cooperate.
The Best Wishes series is infamous for its heavy derailment of Ash himself. After the previous four regions had developed him into a powerful, strategic battler nearly on par with top-level Trainers, Unova rebooted him into a complete rookie who had forgotten even the barest basics. He regresses back to Kanto-era levels of relying more on luck than skill, and it's treated as a complete shock when Ash actually pulls off anything genuinely clever.
Possibly the most egregious example is in his Gym battle against Elesa. He spends hours prior to the battle trying to prepare a strategy for her, only to come up with something completely idiotic: benching his entire team in favor of his inexperienced Palpitoad, expecting to sweep her clean with just one Pokemon. In the end, he chooses to disregard strategy entirely and just rely purely on Pikachu, not considering that the strategy itself might have just been terrible.
This befalls the Team Rocket Trio in Best Wishes as well, effectively meaning that all returning characters in the series (except maybe Pikachu) are derailed in some form. The trio is usually portrayed as comical, bumbling villains, each of them having a developed personality (Jessie is bossy and grumpy but occasionally sweet, James is kind and caring, and Meowth is sneaky and sarcastic) as well as surprisingly deep backstories. They're also often hinted to not really be entirely bad people. In an attempt to make them competent villains, this series throws all of this out of the airlock. The characters barely appear and don't get much focus when they do, and their personalities are extremely diluted to the point of almost being interchangeable, usually being depicted as merely doing villainy and lacking all of their character quirks from previous series. The implication is that they're taking their jobs more "seriously", since they've been promoted, but the personality change is radical and extremely abrupt, making it very jarring. This decision proved controversial and has since been largely reverted in later series.
In mid-Sinnoh, Ambipom is literally Put on a Bus for incredibly contrived reasons that make no sense given the Pokemon's previously established character. From the beginning Aipom/Ambipom was shown to love Contests, which eventually led to her getting traded to Dawn, the Contest Coordinator of the group. DP124 has the characters participating in Pokemon Ping Pong, and Ambipom suddenly decides to leave her trainer and the Contests she loved so much to go off with a complete stranger and train for an activity she didn't even know existed before the previous episode.
Zeta Gundam: Reccoa went from being a fairly capable, trusted member of AEUG with generally straight morals to a TITANS officer who willingly gassed an entire colony. Admittedly she tried to delay it in the hopes that the AEUG would stop it but she never attempted to stop it herself. This after she used the evidence of the TITANS gassing a colony to convince Emma to switch sides at the start of the series. It's idiotic enough that fans prefer to believe that Paptimus has some kind of hypnotic Newtype powers that he used on her.
Gundam AGE gives us Zeheart Galette. For most of the show he's a Noble Demon who makes a point of remembering his fallen comrades and subordinates and fights for every Vagan to return to the "Eden" that is Earth. After Ezelcant reveals his true, Social Darwinist agenda—the one that had him killing his own citizens—Zeheart devotes himself to it after about thirty seconds of hesitation. By the end, he's willing to throw away the lives of his most loyal aides for the sake of a diversion and wipe out multiple Mooks into the bargain. The OVA Memories of Edenexplained this when he admits he objected to Ezelcant's true plan, but went through with it since those who died under his command would have done so for nothing otherwise. It's implied his failure to reconcile these two mindsets led to his subsequent Villainous Breakdown.
And yet the second mode is much more popular. While disturbing, it arguably makes Kuroko a more interesting character. Note that the more exaggerated jokes in the series are not treated quite literally, as while Mikoto will sometimes reduce Kuroko to a smoking crisp in revenge for her antics, she hesitated to use her power against a suit a power armor she assumed had a person in it, and her electricity had realistic and debilitating effects in her fight against Frenda.
Baka and Test: Summon the Beasts did this to poor Mizuki Himeji. In the first season she was a nice, sweet girl, with one scene hinting at yandere-ishHidden Depths but mostly as an one-off gag and nothing serious. Come the OVA and second season, she's basically reduced to that half the time. Thankfully, the last episodes seems to bring her back to rails.
Kanade and Usami are both getting hit by this on Mayo Chiki!. Kanade is going from a sexually dominant, but playful and overall nice girl to a jerkasstroll that has no problem on causing property damage and intimidating others with her Battle Butler. Usami went from a nice tsundere to a generic angry tsundere, overlapping with Character Exaggeration. Kanade eventually gets better, Usami.... is Demoted to Extra so nobody knows.
However, both pale compared to Unlucky Everydude Kinjirou in the Gecko Ending. To summarize, the entire show is about him having gynophobia (fear of women) and the other girls trying to help him with that. Unlike some harem mains, he's not a pervert nor anything like that, and in fact the girls are worse than him. But in the Gecko Ending, he suddenly becomes a very generic pervert that nosebleeds when he sees Usami's panties and can't stop thinking of having sex with Subaru. Which is the exact opposite of how he normally is. He also gets Charles Atlas Superpower out of freaking nowhere.
In Puella Magi Madoka Magica, Kyosuke Kamijou (Sayaka's Childhood Friend love interest, who seems to think his relationship with her as strictly platonic) and Hitomi Shizuki (one of Sayaka's friends and The Rival for Kyosuke's heart) inadvertently send Sayaka on her Start of Darkness. However, their actions in the anime are somewhat understandable and they try to give Sayaka a chance (e.g. Hitomi tells Sayaka to face her feelings and gives her a day to confess to Kamijou). Both Hitomi and Kyosuke were unaware of all the problems that Sayaka had such as Sayaka finding out that her soul got ripped out of her body and put into a Soul Jar because no one tells Hitomi and Kyusuke anything. After Sayaka's death, it is revealed that Hitomi fell into depression and thinks it's all her fault while Kamijou also seems to miss Sayaka. Unfortunately, these shades of gray are all dumped in the PSP game. In a certain route in the game Hitomi becomes a full-onAlphaBitch in Sheep's Clothing, while when Sayaka starts decomposing due to her distance from her Soul Gem Kyosuke calls her "a disgusting (understandable) monster (not so much)", then twists the knife further by adding "Shizuki is the only one I need!" Ouch. However, the outcome of this game does depend on how you play it, so there are some scenarios where Hitomi and Kyosuke don't get character derailment (including a route where Sayaka ultimately gets happiness with Kyosuke).
During his first appearance Obito Uchiha was a guy who strongly believed in The Power of Friendship and could be the potential White Sheep of the Uchiha, but after The Reveal he turns out to be Tobi, a ruthless villain who wants to throw the world into despair and isn't afraid to threaten and harm newborns. And all because Rin died that perceived by some as poorly executed a Freudian Excuse.
Sasuke Uchiha after his FaceHeel Turn showed himself as completely selfish person, who did not care about anything but his revenge (at first Itachi, then Konoha) but after speaking with a resurrected Itachi and a previous Hokage, he suddenly becomes a Well-Intentioned Extremist wishing to organize a revolution in order to become a Hokage similar to how Itachi lived. And all this only in order to eventually lose a fight against Naruto and then change his mind afterwards.
Subaru has the titular character's personality change drastically in the final volume. She becomes snotty, easily angered, and brattier than she had been ever before, despite being 17 at this point. Subaru also begins to insist that nobody but her dead brother Kazuma would ever be able to really understand her, despite part of her arc in America revolving around her realizing that she isn't a solo dancer anymore and part of a group. Her character returns to normal in the sequel.
In Bleach Uryu seemingly has forgotten about how Mayuri tortured his grandfather to death. Instead of displaying open hatred for Mayuri when they meet up again, he only seems mildly annoyed with Mayuri, and his reaction is Played for Laughs. The show seems to be glossing over the first time they met and what Mayuri did to his grandfather and his entire clan. Sure, Uryu in the second meeting wasn't in a position to do anything about his hatred of Mayuri, being too badly injured to even stand up, but if anything that should make him him even angrier.
Yukio and Giriko from the Fullbring arc. They are each calm and quiet normally, but after receiving Ichigo's power, it went to their heads, causing them to become loud and boastful, Yukio declaring himself a god. Yukio, at least, seemed to return to normal after having the extra power taken away, so it may be a case of Drunk on the Dark Side.
In the K-On!movie, Azusa becomes quite upset about the idea that Yui might have romantic feelings for her. Quite a few fans thought this was odd. Azusa might get exasperated by Yui's antics at times in the manga, but it was clear she wasn't against Yui's affection for her in principle, and in the end, even got very close to confessing her feelings to Yui herself. From the other perspective, it can also be seen as derailing Yui from a simple Cuddle Bug to someone who uncomfortably won't take no for an answer.
Tenchi Muyo!'s Mrs. Masaki. From Tenchi's point of view, his mother was this kindhearted woman and it was incredibly possible that Ryoko knew this, too. This characterization would carry over into Tenchi Universe and brief moments of Tenchi in Tokyo under her "Achika" name. Flash forward about six-seven years with the new OVAs and we find out that Tenchi's mother, Kiyone, wasn't a kindhearted woman, but a total prankster who even treated her death as a joke and that the true kindhearted woman was Noboyuki's secretary Rea. Not only were fans pissed off, but so was Tenchi.
There's also Mihoshi, who suddenly loss a massive amount of IQ points between the second OVA series and the third, becoming just as dumb as her alternate universe counterparts.
Goku's wife Chi-Chi is introduced in the original series as a strong, capable fighter in her own right, even convincing Goku to agree to marry her as a result of the fierce fight she puts up in a world-class martial arts tournament. Come Dragon Ball Z, and she's basically abandoned all about her combat skills to become a parody of an Education Mama with severely Skewed Priorities & Insane Troll Logic to boot. While it doesn't excuse her Skewed Priorities, Insane Troll Logic, and some of her other undesirable traits, this may be somewhat justified. This is because dealing with your family members being constantly put in danger will drive the most rational parent insane. It doesn't help that Goku was even killed and Gohan has been kidnapped and brutally injured. With Super, however, she has no excuse. The Buu Arc fixes this somewhat.
Dragon Ball GT did this to Pan. In Z, she's introduced as a bubbly, headstrong 4-year-old girl. Come GT, and she's introduced as a Mouthy Kid and a Bratty Half-Pint towards Trunks and her own grandpa, althought the time difference is long enough for her personality to have changed.
Dragon Ball Super unfortunately reverts Chi-Chi back to her old ways. In DBZ she seemed to mellow out after Goku died fighting Cell, even training their second son Goten in martial arts personally. In Super, not only does she start displaying the same Education Mama attitude towards Goten, but in one episode she is once again, too protective of her family members. She even goes as far as to kidnap her granddaughter away from Gohan because he and Mr. Satan discussed the possibility of her being a martial artist when she grows up.
In Yamada-kun and the Seven Witches, Nene Odagiri is initially, when she actually befriends Yamada and stops just being a drop-in, incredibly loyal and self-sacrifing towards him, not only wanting him to be happy despite her own infatuation, but also going out of her way to ensure he becomes happy. Not that she stops being impolite and hot-tempered - a Hidden Heart of Gold should remain hidden in her eyes - but she is A Friend in Need, and he comes to consider her so. When the problem is solved, and Yamada gets together with the girl he loves - Shiraishi - she becomes much more mean-spirited. Odagiri has now suddenly forgotten that she wanted him to be happy and tries to steal Yamada away from Shiraishi. She still helps him out when he needs her, but usually only does it with the intention of having her notice him, and she will pull every trick in the book to convince him that he should rather associate with her than Shiraishi.
The last few episodes of Le Chevalier d'Eon do this with Robin. He goes from being friends with and saving D'eon's life to indifferent and willing to fight him, while instead teaming up with the main villain for the series. They tried to write in reason for it... but just didn't sell it very well.
For the first three seasons of Cardfight!! Vanguard G, Chrono is very empathetic and responsible, and always trying to take care of others, even at his most standoffish. He values honesty and openness greatly - he routinely shares everything he knows about whatever plot is going on with his friends and teammates, and hates being kept in the dark by others himself. Come season NEXT, he's suddenly extremely pushy and oblivious to other's feelings, keeps plot-related secrets from everyone including his own team, and prioritizes winning a card game tournament over actually saving someone from a forced possession by the season's Big Bad.