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.
Yu-Gi-Oh!: For a good number of people this happened in the filler arc, 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 playing a card that would go against his value system even so. Happens to Kaiba throughout the anime to a lesser extent, as he continually forgot lessens about teamwork.
Yu-Gi-Oh! 5D's: 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. To clarify: The only thing the former Aki and the recent Aki have in common are their physical attributes.
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. It's like they completely screwed up the chance to show a less invulnerable, more three-dimensional Prime.
Nina Wang got hit with this hard after Episode 16 of Mai-Otome. Before, she was a somewhat friendly rival to Arika and a more-than-competent fighter on track to become one of the greatest Otome ever. Afterward, she underwent a Face–Heel Turn and killed tens of thousands of people to prove her loyalty to/love for her "father", and her rivalry was thrown on the back burner for all but one or two episodes during that period, and her position was snapped up by Tomoe for reasons unknown to Arika.
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 Face–Heel 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 those events, he became a regular psychotic baddie whose sole purpose in life was betraying Tempura in ever way possible and becoming royalty.
Ash's Charmander, upon evolution into Charmeleon, went from nice compared to Ash's other Kanto starters to volatile and uncontrollable. Upon evolution into Charizard, it went even further to a borderline jerk that actually seemed to dislike Ash until halfway through the Orange Islands arc because Ash spent a lot of time in effort to keep it alive. It was probably supposed to be similar to Pokemon above a certain level refusing to obey their trainers without a certain badge in the games, but stop even after Ash had gotten all eight badges. The show can't seem to decide whether or not evolving changes who you are (Charmander's personality changes, Pikachu and Bulbasaur directly refuse to evolve) or if it changes your form but nothing else (most other evolutions including a Magikarp that retained its sweet personality for the Joy that looked after it).
Brock was the voice of reason with occasional attractions towards girls in Kanto. When he got back from his bus trip during the Orange Islands Filler Arc, he became a super freak with raging hard-ons every time he sees things with two legs and a vagina. And his voice of reason status has apparently been reduced to just explaining to his comrades (and the audience) every single move that gets made during a battle. The writers caught on by the end of the Sinnoh arc and gave him a mercy bus ride.
Considering that Ash has battled various opponents in the past with impressive feats, you'd think he'd be able to beat Paul or Trip without getting his ass handed to him. It was explained a few appearances in that Paul is pretty much just as experienced as Ash is. And while that is admittedly an Ass Pull, there is still no question he is smarter and a better tactician compared to Ash. Trip on the other hand, there is no excuse whatsoever that Ash should lose, or even have trouble with a beginner trainer, at all.
Paul's transition from Jerk Ass to a more considerate and caring trainer at the League tournament was way too abrupt and made little sense, with little to no explanation to it other than being used to finally put an end to his rivalry with Ash.
In mid-Sinnoh, Ambipom was literally Put on a Bus, for incredibly contrived reasons that made no sense given the Pokemon's previously established character. From the beginning Aipom/Ambipom was shown to love Contests, which eventually led to it getting traded to Dawn, the Contest Coordinator of the group. DP 124 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 lead to his subsequent Villainous Breakdown.
A case can be made for more than a few of the characters of Code Geass, but none more so than Ohgi, initially the voice of reason of the rank-and-file Black Knights who understood the burden of leadership that rested upon Zero, he lost the plot once he fell for Villetta and made many questionable decisions, with ultimately disastrous results.
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 a clone of the the much-ruder Minami (who herself suffered flanderization and became much worse than she was) only with longer hair and bigger breasts. Thankfully, the last episodes seems to bring her back to rails, while also smoothing over Minami a little.
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.
Tails from Sonic X. In the first two seasons, while there was some heartwarming bonding between him and Chuck, he was characterized as being someone extremely intelligent who happened to be a kid, rather than a kid who happened to be extremely intelligent as he is characterized in the games. He becomes derailed even more in the third season.
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 Face–Heel Turn showed himself as completely selfish person, whom do not care about anything but his revenge (at first Itachi, then Konoha) but after speaking with resurrected Itachi and previous Hokage, he suddenly becomes 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 losing a fight against Naruto and then change his mind at no reason.
Before Madara's resurrection, Black Zetsu is shown to be a loyal servant of Akatsuki and Tobi spawned from from Madara's will who is willing to do whatever the latter needs to do with absolutely no indication of a scheme or treachery. In fact, when he forced Tobi to resurrect Madara, he actually apologize for being forced to do it. But when Madara is revived, Black Zetsu all of a sudden is revealed to be just as callous as Madara who berates Obito for not following Madara's plans. To add the derailment even further, at the turning point of the story, he backstabs Madara revealing that all along he is in fact, a will of Kaguya, despite previously having virtually no indication that he worked for Kaguya in the first place.
Subaru has the titular character's personality change drastically in the final volume. She becomes snotty, easily angry and the brattiest 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 lose 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 OV As 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.
In Dragon Ball, Goku's future wife Chi-chi is introduced 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 artist tournament. Come Dragon Ball Z, and she's basically abandoned all about her combat skills to become a parody of an Education Mama with severly 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 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.