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.
Brock from Pokémon 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. The writers caught on by the end of the Sinnoh arc and gave him a mercy bus ride.
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.
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 once without getting his ass handed to him. No matter what, post-Sinnoh Ash seems to fight like an inept beginner, especially considering his impressive victories in the past; and don't get anyone started on the ass whipping he gave to Fantina in a rematch.
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 in the heat of the moment 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.
On that note, 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. Many fans are not happy.
The first example would be Ash's Charmander, which upon evolution into Charmeleon, went from similarly nice compared to Ash's other Kanto starters to volatile and uncontrollable. Upon evolution into Charizard a while later, it went even further to a borderline jerk that actually seemed to dislike Ash. Charizard's attitude even cost Ash his match at his first Pokemon tournament. 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 it firstly still came off as odd, and secondly didn't stop until after Ash had gotten all eight badges, competed in the Kanto League, and gotten halfway through the Orange Islands arc... And even then only because Ash spent a lot of time in effort to keep it alive.
Made worse in that 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).
Gary Oak as well. In the early seasons, he looked to be a competent rival to Ash, and fans couldn't wait for the confrontation between them...which didn't happen for five years, by which time Seasonal Rot had driven off more than a few viewers. After this, he became a Pokémon researcher, which has been his position ever since.
What in God's name happened to Graham Aker in season 2 of Gundam 00? In season 1 he was an obsessive Flag pilot, and totally loyal to The Union. Then in season 2 he's... an obsessed Japanese Otaku calling himself Mr. Bushido? Who acts like he just walked out of a Super Robot show? What. The. Hell. And the fanbase that decries the situation in Seed/Seed Destiny says nothing about it? Yes Graham's transformation is funny. The first time Mr. Bushido appears it's hard not to laugh at how hysterical he's become (Tropes—even this one—Are Not Bad after all). But it's still Character Derailment.
His derailment does have a partial explanation. He shows small hints of his otaku ways all throughout Season 1, but he's otherwise normal. In Season 2, whenever he's not fighting he does have moments where he does act normal, even telling Billy the "Mr. Bushido" moniker was not one he was purposely angling for, and when he meets up with Setsuna it's flatly obvious the man has been severely depressed for awhile and he reveals he immersed himself in his Japanese otaku ways as something of an escape from the pain. Unfortunately, the derailment arguement still holds water because the show does a poor job of explaining the whys and when of his plunge into Occidental Otaku.
Nena Trinity got derailed during the 2nd season, as well. After beginning to develop standards and generally become less psychotic with an implication that she might actually regret the horrible things she did now that she's gotten a taste of what it's like to be on the receiving end, a lengthy timeskip occurs and Nena is suddenly right back to her previously evil, batshit insane self with no explanation whatsoever given. And then she dies.
Gundam AGE gives us Zeheart Galette. For most of the show he's a Worthy Opponent 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. Perhaps it's the failure to reconcile these two wildly different characterizations that leads 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) are probably the most hated characters in the series(after Kyubey) for sending Sayaka on her Start of Darkness. However, their actions in the anime are somewhat understandable and they do give Sayaka some Hope Spots (e.g. Hitomi tells Sayaka to face her feelings and gives her a day to confess to Kamijou). Additionally, 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. In the last few episodes, 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. Hitomi becomes a full-onAlphaBitch in Sheep's Clothing, while Kamijou, Sayaka start decomposing due to her distance from her Soul Gem and 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. But, the derailment they get in some of the scenarios is still pretty bad.
In Naruto, Obito, at first a guy who strongly believed in The Power of Friendship and could be the potential White Sheep of the Uchiha, becomes Tobi, a ruthless villain who wants to throw the world into despair and isn't afraid to threaten and harm newborns. All because Rin died. Hand-waved in that the same Bloodline Limit that allows the Uchiha to unlock Sharingan also affects their brains, making them go crazy when someone they love dies.
Sasukevowed to become a Hokage similar to how Itachi lived only to change his mind at the end for no reason other then losing a fight against Naruto.
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.
Zeta Gundam's Reccoa. She 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.
Similar to the Fire Princess Ursa of Avatar: The Last Airbender example in the next page, we have 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 forgotten all about her combat skills to become a parody of an Education Mama, one who piles on tons of homework for her four-year-old son and who at one point tries to prevent her son, one of the world's strongest fighters, from training because it will cut into his studying time. The training is for a battle that will prevent the destruction of the entire planet. And when this is pointed out to her, with the unspoken yet obvious implication that there won't be a need for Gohan to be a scholar if the entire world is dead, she retorts that she couldn't care less about the world, only her son's future.
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.