Luanne from King of the Hill started out as a somewhat promiscuous teenager who didn't take crap from anyone and, despite being Book Dumb, had some intelligence (the episode that fully introduced Hank's father, Cotton, revealed that Luanne can fix a car, despite Cotton's "Stay in the Kitchen" views on women). Later episodes turned her into a shy moron who was afraid to stand up for herself (and when she married Lucky, her naivete and lack of intelligence were greatly exaggerated to childlike levels). The promiscuity, however, was given a proper episode (where Luanne joins a group of promiscuous people who want to be born-again virgins) when it disappeared.
Bobby Hill was similarly much less effeminate in his early years. It could be argued as Character Development though if you didn't take well to his original characterization as a "Well Done, Son" Guy who's slow, awkward, and, in the words of Hank Hill, "not right." Sure he got effeminate in later episodes, but he got also got some charisma and charm, even if it was limited to being a class clown. He also went from a shy, reclusive fat kid to someone who has friends, a girlfriend, is sure of himself, and has proven to both Hank and COTTON (the episode where Cotton takes over his military school, and goes one day longer in "The Hole" than Cotton managed) how much of a badass he is. All while not changing himself to fit how they think he should be.
On the other hand, some consider Cotton to have undergone a case of derailment during his final appearance, which retcons his grudge against the Japanese despite him having forgiven them in an earlier episode, talked out of his scheme to spit in the Emperor's face by his illegitimate son Junichiro. This gained the episode criticism from some viewers since while Cotton was not well liked, the way he forgave the Japanese was considered development.
And don't even get started on Peggy.
Hank had also undergone Derailment as the show went on. During the first few seasons he was shown to enjoy Rock and Roll and practiced Guitar at times and open to new things. By the end he was a stuckup tightass who was so old fashioned you'd think he was Amish and views things out of his comfort zone with contempt and disdain.
Although that's a fair cop for Megatron's unprecedented manias, it's arguably justified for the Maximals, given the impact of the massive shock of rapidly going from having won the Beast Wars with Megs as their captive to suddenly finding themselves mode-locked, downgraded, amnesiac and on the run from a massive army of drones amidst the newly post-apocalyptic landscape of their homeworld after Megatron arrived ahead of them at an earlier point in time and conquered Cybertron before they ever got there, and realizing that the entire population have had their sparks ripped out and stockpiled. Planetary genocide is a pretty good motivation to shake your personalities up a fair bit, so for some viewers this was more a case of justified (if not necessarily welcomed) Character Development.
Honestly, people have turned into religious fanatics over much, much less than what Primal had to deal with (not only has his entire race been wiped out, he's indirectly responsible).
Actually, Rattrap was probably the worst offender. In Beast Wars he hated the 'cons more than any of his fellow Maximals and was incredibly resourceful in combat situations (he even had an entire episode, "A Better Mousetrap," dedicated to his resourcefulness), but in Beast Machines he made a deal with Megatron himself, a guy he'd already loathed even before he decided to wipe out their entire race, just because he found out his new body didn't have weapons. Thankfully they got him somewhat back on track after that, having him fight by improvising or building weapons like the plant grenades, but that episode was still pretty jarring.
Then there's Silverbolt. Because the time he'd been programmed into the awesomely evil Jetstorm had been enjoyable on some level, he spends the rest of the series wracked with guilt and having nothing to his personality beyond it... more than any of the others, he's Silverbolt In Name Only. The writers said they'd planned for it to be a growth from the Adam West Batman of Beast Wars Silverbolt to the Dark Knight of the Beast Machines Silverbolt, but that's not what we got at all. If you want to put it in comic book terms, what you really had was Speedball into Penance. Worse was the fact that Jetstorm was the Ensemble Dark Horse and an all-around fun and dangerous villain, so it's two beloved characters we lose to the Bleedball version of Silverbolt.
Rhinox is possibly the biggest derailment of them all. Once he's brought to the surface and put in control of Tankor, it is revealed that he agrees with Megatron. From the level-headed, spiritually-minded second in command of the Maximals, who actually enjoyed Earth's natural environment, to a self-stated Beast Mode-hating supporter of Megatron.Great job, writers. Real top notch work there. It was intended for Rhinox to be a lot more reasonable about it, explaining exactly why he thought this now, but Richard Newman's delivery of the lines made him sound like he had gone insane. A lot of fans pretend that the Vehicon programming was still affecting him, and it's also worth noting that he attempts to betray Megatron after this revelation.
It's not just Edward that this has happened to. Percy went from a mischievous Cloud Cuckoo Lander to a naive, forgetful moron who can't pronounce simple words, Toby was a kind and confident Cool Old Guy but later became a wimp with self-esteem issues, and Skarloey, previously one of the oldest and wisest engines on the island, is now childish and afraid of everything (from thunderstorms to the incline yards to the wharf). Thomas himself was once depicted as friendly and helpful, if somewhat cocky and brash. He's now a poorly-written Creator's PetStu that is obligated to make appearances in every single episode (even episodes about the narrow gauge engines!) and dispense advice to everyone.
Some derailments have been taken a different way in the CGI series, either reversed back, exaggerated or taken a different way altogether. Skarloey is back to his more mature and no nonsense persona, while Edward is kind again, albeit to a fault, unable to say no to others and easily dissuaded from his work. Thomas interchanges between his older and newer persona to erratic extremes while Toby's meekness has reached Fluttershy/Chuckie Finster levels, afraid of nearly anything that moves.
Thankfully though, starting with Season 17, which came with a brand new writing team determined to return the show to its roots, almost everyone of the characters above have returned to their original selves.
Diesel 10, who currently provides the page image, is a very unique case of derailment. In his debut in Thomas and the Magic Railroad, he was a murderous monster who was out for blood, wanting to completely and utterly destroy the steam engines. In his second appearance in Calling All Engines, he's somehow become nicer with almost no given explanation, and quite willingly helps Thomas repair the new Sodor Airport. In his defense, it was related more to necessity since a decline in tourists brought in by the airport in question would've spelt doom for the entire railway, including him. Even so, he doesn't display any sort of gruffness or malice, and again, does it with a big smile on his face! However, when brought in for the CGI series, he became more of a mix of the two. Whilst he certainly wasn't nice like he was in CAE, it wasn't up to his TATMR persona, as he's now become more of a Well-Intentioned Extremist in that he wants his fellow diesels to receive more praise and attention than what they have, believing that they've become overlooked in favor of the steam engines, and while he certainly doesn't LIKE them, he does realize when he's gone too far whenever he deals with them. It has given him more of a personality though, as some felt that his TATMR appearance was more of a Generic Doomsday Villain than an actual character.
Pam in Archer became a completely different character as the series went on. Starting off as a pleasant but lonely and depressed HR rep, she turned into a sex-crazed, drug addicted complete loon. Though as always, Tropes Are Not Bad, as many fans enjoy this version of Pam much more.
Vilgax went through some change as well; Alien Force portrayed him as Lawful Evil (though it last only one episode and it's strongly hinted he was actually posing as such for his own advantage; in later episode, he has no problem breaking laws); later, Ultimate Alien portrayed him as a Manipulative Bastard, which, while being some derailment in a way, was actually good and ended up being an Author's Saving Throw after the Badass Decay he had suffered in Alien Force.
The Simpsons is a major offender: Homer going from a well-intentioned buffoon to a cartoonish idiot to a Jerkass who has little empathy for those around him, Marge going from a sensible, down-to-earth woman to ditzy, snappy housewife, Bart going from an underachieving prankster and troublemaker to a full-time delinquent, and Lisa going from an idealistic child genius to ultra-liberal, activism-obsessed, mini-teenager.
Maggie turned out pretty good, though. She went from cute baby with a pacifier to cute baby with pacifier who can fire a gun!
Flanders, when he first appeared, was a perfectly nice guy. Yes he was religious and more than a little repressed. But he wasn't hurting anyone and he seemed to love all people unconditionally...in fact the whole JOKE was that Homer hated him for NO reason. He was the kind of neighbor most people would be lucky to have...friendly, kind and always willing to lend you whatever you needed. Then came his Flanderization, where he went from a generally nice guy to his ludicrously meek, inoffensive, and Christianity-obsessed persona. The derailment kicked in later, when the Simpsons writers decided they wanted a Right-Wing Christian Strawman to mock and so they derailed Flanders to make him fit the bill.
And maybe this is more Early-Installment Weirdness than character derailment, but in Flanders' very first appearance he was so thoroughly insensitive to Homer's obvious financial troubles that he came across as a complete Jerkass fully deserving of Homer's enmity (though Ned may not have been aware of just how bad Homer's financial problems were).
In fact, some recent episodes show that the derailment has effectively inverted the Ned-Homer dynamic, as now Homer is a happy-go-lucky idiot with Ned loathing him effectively just for existing.
C. Montgomery Burns went through Villain Decay like hell. In early seasons, Burns was a downright sadistic and evil old miser who generally treated his employees like crap, stole from children, tried to block out the sun (for money), and at one point planned on turning a litter of puppies into a fur coat. Turn to later seasons and he's just a feeble old man who is more ineffectual than anything else and serves more as fodder for jokes about being behind the times then anything else.
Nobody got it worse than Patty and Selma. They were originally just a couple of gruff curmudgeons who hated Homer and think Marge should divorce Homer, take the kids with her, and remarry. Later seasons would add appalling housekeeping and near-criminal incompetence at their jobs (both arguably even worse than Homer's), not to mention smoking everywhere, including around children and on the job at the DMV (which nearly gets them fired — until Homer takes the fall for the both of them). Selma would go through three astonishingly bad marriages (and she only wised up after realizing how long her last name was), while Patty became a full-fledged militant lesbian (despite that "Principal Charming" established that Patty is heterosexualnote though the "Treehouse of Horror" story with the killer Krusty doll had her declare that she became a lesbian after she saw Homer run naked through the kitchen and willingly celibate, while Selma has had celibacy thrust upon her and wants to have a husband and kids before she dies alone and unloved). Of course, all this pales next to becoming utterly obsessed with trying to end Homer's marriage, including celebrating his death by buying a tombstone for Marge, trying to kill him by pushing him off a bridge (though that was part of Homer's plot to land on the boat where his "Congratulations on Completing Community Service" party is being held. Still the fact that Patty and Selma were willing to kill Homer just because is unsettling)...and attempting again later by reenacting the famous bathroom torture scene from the first Saw movie.
Quite a few characters in Family Guy have also warped considerably since their first appearances, especially after the show's return to production in 2005. Among the many examples, Stewie, previously a humorously sociopathic Enfante Terrible with matricidal tendencies, now seems to simply be Brian's effeminate sidekick (though his violent tendencies do occasionally show up), while Brian has changed from a snarky intellectual ironically portrayed as vastly more intelligent than his owner Peter, to mainly a preacher for the writers' liberal political views and The Chew Toy concerning his inability to quit drinking, be a good writer, and hold a relationship. Lois, while at first as the down to earth one of the family, albeit with a somewhat healthy sexual appetite, has become pretty much an insane nymphomaniac. Meg has evolved from merely a unpopular high-school girl trying to fit in to an abused, suicidal psycho of epic proportions. Peter himself, while always a bumbling idiot but with slightly good intentions (like Homer Simpson, only fatter, drunker, and more into pop culture from the 1970s and 1980s), has become a Jerk AssKarma Houdini (making Homer Simpson look like the clueless, yet caring Bumbling Dad he was in the early days of The Simpsons), though this has been toned down somewhat in later episodes. Around Season 11, Chris Took a Level in Jerkass just like his parents.
Not to mention recurring characters like Cleveland who often dispensed simple wisdom and often kept Peter in check during some of the wilder schemes. Quagmire went from being a Casanova Wannabe to a sex-obsessed freak (though "Jerome Is The New Black" did have a Freudian Excuse for that: Quagmire was once in love with Cheryl Tiegs. Unfortunately, Cheryl Tiegs left Quagmire before he could tell her that, and since then, he's been addicted to sex to fill that void she left), Joe became overly macho to compensate for his handicap as opposed to his original character which was more like a good cop who happened to be crippled, and Mort, originally just a Sickly Neurotic Geek, was reduced to a caricature of nearly every Jewish stereotype you could imagine (despite not appearing much in the later episodes).
Quagmire seems to be undergoing an unusual second derailment in recent episodes, with his intense (and randomly produced) hatred for Brian and Self-Deprecation based neuroses becoming the focus of a lot of his humor.
The Cleveland Show episode "A Rodent Like This" reveals that The fat, weird, slow Cleveland Jr. is an imposter who killed the original and took his place and nobody noticed because they were too stupid though that may have been a simulation.
Speaking of Cleveland Jr., just like most of the Griffin family he Took a Level in Jerkass during the show's final two and a half seasons.
Lois' character changes could be at least somewhat justified. Being married to and living with a man like Peter for years could easily drive the most sane, sensible person crazy. One episode even implies this by showing a tumor in her brain after she mentions bottling up all the stress that Peter causes her.
Before getting his own show, Cleveland had been reduced to little more than an excuse for the writers to use as many black jokes as possible whenever he was on screen.
Despite mass Flanderization, most of the characters on SpongeBob SquarePants retained some level of their original personalities. Plankton however started out as a competent, aggressive and maniacal villain. Once The Movie rolled around he lost much of the latter in favor of being either too obnoxiously happy with the few moments he seems to succeed, or constantly moaning and crying about what a failure he is. He's also lost a lot of competence, to the point that his Deadpan Snarker computer wife often has to tell him how to do every basic step (even blink) with Plankton often taking credit for her suggestions.
SpongeBob himself may have gotten the worst of this. He started out as a naive, yet kindhearted guy who cared about everyone no matter what, and had the innocence of a child, to an obnoxious, nosy Man Child who pesters everyone he knows to play with him, he is often careless of others' safety, and cries and screams when his "friends" aren't around.
Patrick got progressively dumber as the series went on, going from average intelligence in the first season to pretty stupid in season 2 and 3 to virtually brain-dead in season 4 and onwards, he also became more of a Jerk Ass on occasion.
While the fanbase seems completely divided whether Daffy Duck'sFlanderization during the 1950s from a bombastic Cloudcuckoolander into a JerkassSmall Name, Big Ego counts as derailment or development, most agree it crossed the line during the De Patie Freleng era shorts, where he was evolved into a humorless, Faux Affably Evil villain for Speedy Gonzales. One could take it as a double-sided coin, as pitting Speedy against a more conniving villain did at least allow him to come off as more sympathetic and fallible more often than he did against Sylvester, though it still would have been a swifter transition with a more fittingly-callous character.
He's even worse in The Looney Tunes Show while he does have of his Cloudcuckoolander and Jerkass moments he is now a total dumbass, he can't read or write, he can't answer even the simplest trivia questions correctly, he doesn't understand the concept of stealing, doesn't know the concept of time, etc.
To elaborate, Bloo in the pilot and some of the earlier episodes is a friendly and well-meaning but mischievous imaginary friend who likes to have fun. In the rest of the series he's just a Jerkass who will do anything to get what he wants and loves being stupid and breaking the rules.
In the first few episodes, Bloo was still sort of the nice but mischievous character he was in the pilot, but was driven sort of neurotic by Herriman's obsessive rules. Even during these episodes, he seemed to be subtly shifting toward being ruder and ruder.
Also, Madame Foster. According to everyone else on the show, she's supposed to be the sweetest (and coolest) grandma ever and an absolute saint of a human being; in some episodes, she does live up to this reputation. Yet in the infamous Season 3 episode "Foster's Goes To Europe", she ruins a European vacation which the gang planned to go on (and spent the entire episode struggling to get ready for) by stealing eight-year-old Mac's tickets at the very last minute. The episode ends with Madame Foster, her bowling buddies and a homesick imaginary friend called Eurotrish going on the vacation instead, while poor Mac is wrongly and unfairlyblamed by everyone else for missing the trip. In the same exact episode, Madame Foster also unties a group of defenseless imaginary balloons which sends them flying into the sky (and inevitable demise). For no reason at all. None. Wonderful lady indeed!
Numerous examples after the first season of the Total Drama series.
Trent from Total Drama Island to Total Drama Action. Trent started out in TDI as the closest thing to a normal teenager: sweet, smart, nice, and talented. He develops a thing for Gwen and they end up going out on the last episode. By the fifth episode of TDA, his crush on Gwen has turned him into a desperate loser who has to please her since he's so worried about her feelings dwindling since she has more in common with her fellow teammate Duncan. It gets so bad that he starts throwing challenges for her and Gwen has to convince his team to boot him off.
Geoff and Bridgette (or 'Gigette') lost both their personalities once they became an Official Couple and were only used for make-out gags, Geoff suffered from Acquired Situational Narcissism during the Aftermath segments and Bridgette ended up becoming a Clingy Jealous Girl during the TDA special and even cries when she realizes that Geoff won't be competing alongside her during Total Drama World Tour.
Courtney went from a whiny but tolerable Small Name, Big Ego who had a sense of honor in playing the game to a haughty, manipulative, Jerk Ass who sues if she doesn't get what she wants (something she only did once in Island, in a justifiable case) and is pretty much as bad - or worse! - than Heather was.
Thankfully, as of the special, she's snapped back a bit, and gotten a more realistic plot of her love triangle with Duncan and Gwen.
Aaaand it seems they've driven her right back off the rails after Duncan and Gwen kissed, most likely done in order to kill DuncanxCourtney dead. Being furious and upset over the incident at hand is one thing, but actually losing challenges on purpose to spite Gwen? Good lord, what has happened to the ultra-pragmatic Courtney from Total Drama Island!?
Things just get worse for Courtney in All Stars. At first it appeared as if the writers were finally trying to get Courtney's character back on track. After the whole Duncan incident, Courtney finally became Gwen's friend again and the two were just as close as ever, and she even pursued a new romance with Scott. It looked as though Courtney finally had a chance to be likable again. All that development was suddenly derailed in one entire episode. Out of nowhere, Courtney reveals that she has already planned out what she's going to do now that she's in the final five and what order she plans on getting rid of the other contestants, two of whom are her close friend and new love interest. She chooses to back-stab them because a million dollars is worth more than her relationship with either of them apparently.
Gwen on Total Drama World Tour does an example of Moral Dissonance by kissing Duncan while he was still with Courtney. She feels some remorse at first but when the kiss is found out she doesn't try to vote herself out like last season or apologize. And later she manipulates Cody's feelings for vote Courtney off. She's even lampshaded as being "New Heather".
Leshawna, most definitely. The first season had her as a girl who would take crap from nobody and could get quite feisty, but still very kind and friendly, to the point where her "elimination" came in part because everybody who had been eliminated at that point liked her so much. Then comes TDA, where she does manipulate her teammates into giving her a reward by pretending to cry and being sentimental. And here comes World Tour, in which she's somehow become a huge, arrogant jerk with little to no redeeming qualities, and goes right back to hating Heather even after supposedly making up with her in TDA; even beating Heather up when Heather is trying to warn her about Alejandro's treachery and, after learning that she was right, is still proud of doing it.
Duncan also qualifies because during his return in World Tour he seemed to hold the Conflict Ball, he cheats on Courtney with Gwen, he never helps his team, cheats during the game, treats everyone (other than Gwen) like crap, especially Courtney, became a Manipulative Bastard and helped Alejandro stir the pot.
Owen also counts. He started out as a friendly and completely clueless big oaf who everyone loved and who didn't play the game strategically. In TDA, he becomes a self-centered Small Name, Big Ego and Jerk Ass who can't stop himself from eating for one second. Later comes TDWT where he has dropped the Small Name, Big Ego and Jerk Ass traits but now he actually THINKS STRATEGICALLY.
From the new cast, we have Mike, Zoey, and Cameron. Ignoring the fact that this trio are some of the show's biggest screenhogs, they also went downhill during All-Stars. Mike got a very unpopular subplot taking place inside his brain, Zoey became Too Dumb to Live by not realizing Mike was actually Mal despite being warned by three different people, and Cameron was saved from elimination twice for no good reason, not to mention the fact that he also stupidly trusted Mike.
Go watch Cosmo in the earlier seasons of The Fairly Oddparents. It was established in the first season that he ran away from home to marry Wanda. In another episode he broke down from having to be away from her overnight. It was very clear they enjoyed their relationship. Then, suddenly, in seasons four and five he was drooling over every attractive woman that came onscreen, repeatedly calling Wanda a nag and more, and acting like she had Bound and Gagged him at the altar. Hell, go watch the original Oh Yeah! Cartoons shorts of Fairly OddParents. Compare the calm, deeper-voiced Cosmo who was Wanda's equal intelligence-wise and actually seemed to share a loving relationship with his wife. Now, compare that to the sociopathic Man Child with the screechy voice portrayed in the Fairly Odd Parents series and see how far he fell. It's rather handy, you can tell his intellect by his voice pitch. Deep/slightly high voice, reasonable degree of smarts, really high voice, no smarts.
Remember when Wanda happily admitted to, along with Cosmo, being "two halves of a whole idiot?" Now she's smart and Cosmo is just one whole idiot.
Remember when Vicky used tapes, recordings, personal belonging-destruction and other tiny bits of humiliation to torture Timmy? Now she's armed to the teeth with chainsaws, maces, and other over-the-top weaponry with her cackling maniacally all the while. This derails Vicky from far-back as the first episode, where she clearly shows she'd never want Timmy to suffer any serious physical harm, let alone be the cause of it, because her well-paying job depends on him being kept in-tact.
Even Timmy himself technically counts. He started off as a typical frustrated 10-year old, a bit of an outcast, and occasionally selfish. He always learned his lesson if he made a mistake, though, and only made wishes whenever they were issues crucial to his life. Then later he became more self-centered and wishing for stuff he wanted. Though season 6 and onward more or less reverted Timmy back to how he was in the first season.
Timmy's parents are an interesting case. When the series first started, they were portrayed as tired and overworked from their jobs, but they still genuinely cared about their son. Their biggest crime was that they were more than willing to lie to Timmy so they could get time to themselves; however, they both seemed to learn their lesson in "Abra-Catastrophe." But then...something happened. As the series went on, we learn that Timmy's parents were expecting a girl (hence his pink hat), and in the infamous "It's a Wishful Life" episode, it's shown that Timmy's parents would have a child actress for a daughter and would be rich if Timmy had never existed. Then, both of Timmy's parents started making comments about how they could rent out his room if he went to military school, and even casually admit that they sometimes forget to feed Timmy. And that's not even getting into how Timmy's dad, like Cosmo, was turned into an idiotic Man Child. Sure, Timmy's parents have been able to deliver the occasional Crowning Moment of Heartwarming in later seasons, but there's no denying that somewhere along the line, they went from busy, albeit genuinely caring parents to borderline Abusive Parents who often cross the Dude, Not Funny! territory.
Considering all of its faults, many agree that one of the greater atrocities committed by The Secret of NIMH 2 was entirely derailing the character of Brutus; changing him from a white-eyed, silent, spike-wielding gatekeeper into...a blundering, idiotic side-kick to the also-somewhat-derailed Justin.
It should be noted that the original book Ms Frisby and the Rats of NIMH did actually reveal Brutus to be a pleasant guy just playing the shadowy guard act to scare away intruders. Similar to the Aunt Shrew example, it is more the fault of both films for not elaborating on this enough.
Or Aunty Shrew scolding Martin for badmouthing Nicodemous and insisting that he was a great oracle or something - despite the fact that she clearly was shown in the first movie distrusting the rats (though it's possible she thought better of them after they helped the Brisby family, there really isn't any mention of this).
The first Tom and Jerryanimated movie not only had the title characters (who are supposed to be, you know, pretty much mute) talk... it had them sing. And be friends. You know, the same cat and mouse who are supposed to be bitter enemies are singing about friendship.
The Character Derailment actually happens within the movie itself, as Tom and Jerry spend the first ten minutes of the movie as their normal, mute, bitter enemy selves (though Tom does save Jerry from the collapsing house, it feels more like one of the Enemy Mine moments they'd occasionally have in the original shorts), until the dog convinces them to be friends. For about a minute, and their first bits of dialogue, they seem to cling desperately to their characterizations as enemies, as well as engaging in some admittedly pretty funny lampshading of the sudden shift:
Tom/Jerry: You talked!!
Tom: Well, sure I talk! What do you think I am, a dummy?
Jerry: You said it, I didn't!
Tom: Ah, you little pipsqueak! I oughtta — *pause* Hey! How come you never spoke before?!
Jerry: Well, there was nothing I wanted to say that I thought you'd understand... and there STILL isn't!
Unfortunately, shortly after this they're goaded into singing the Big Friendship Song, which marks the definite moment when the Character Derailment can no longer be denied even by the most good-willed fans.
This goes back to 1975 when Hanna-Barbera rebooted Tom and Jerry for ABC Saturday mornings. They started out antagonistic in the first episode "No Way, Stowaways," only for them to shake hands at the end. As the series went on after two or three episodes of going at each other (without the old films' violence), they remained bosom buddies.
Scott Summers/Cyclops on Wolverine and the X-Men. In his origin episode Breakdown Cyclops, who has been depressed and ineffectual since losing his girlfriend Jean Grey, is revealed to have been extremely stupid and clumsy when he was first recruited to train as an X-Man. This apparently changed when Jean came into his life and "took the pain away" as Emma Frost, the rival love interest, put it. This is a big turn from the comics, where Cyclops was always a good, diligent X-Man, even as a student, where his primary problem was that he had weak social skills. This alternative characterization however embraces the stereotype that he is nothing without his long time girlfriend, while not showing anything suggesting he was ever a competent heroic leader.
Hell, lets say EVERYONE was derailed to some extent, literally the only exception is Nightcrawler, they all become massively incompetent, just to make Logan's Canon Sue aspects all the more apparent.
Granddad on The Boondocks: In the first season, he was one of the two straight men, generally went his own way and couldn't stand his grandsons insufferable Huey or no-good trouble-making Riley and didn't hesitate to take off his belt and give Riley an ass whooping whenever he had it coming. He's now a full-on Jerk Assabusive grandparent who wants to seem young and hip, obsessed with money & "bitches", and blindly follows along with whatever dishonest schemes Riley has planned to get rich quick, over Huey's loud and sensible objections. Granddad over the course of 3 seasons came to embrace all of the black stereotypes Aaron McGruder has been making fun of (when it was just Riley).
Characters in the PBS show Arthur would fall victim to this in later seasons. It is especially poignant with episodes written by Dietrich Smith, though it varies depending on who writes. This is especially problematic, particularly among older fans, as Arthur is that kind of show with an established continuity and set character personalities.
Their dynamic also derailed in some form due to the Urpneys decaying. In the first episode, the Urpneys were pitiful unwilling bumblers, but they and Zordrak were still effective enough to put the heroes in genuine mortal danger, allowing Rufus to come off as sympathetic against them. In most later episodes, they are completely ineffectual and doomed to failure from the start, with the heroes gaining a smug awareness of this and often taking fun out of tormenting the Urpneys just to ease the monotony of their routine schemes. In one episode, the Noops outright retreated in terror from the Urpneys because they had become too close an actual threat, nearing them more as self righteous Miles Gloriosuses who are only willing to bully harmless targets.
At least some attempts were made to remedy both cases of Derailment throughout Season Three and Four, giving the Noops more Character Checks to their old personas and reverting them to competent but pragmatic heroes against the Urpneys (along with giving the villains a more Not So Harmless motive so that the heroes came off as genuinely defending themselves).
EVERYONE in the Watership Downseries adaption. Everyone. Hazel is suddenly in love with Primrose after about two episodes, Hyzenthlay was completely replaced altogether. Bigwig, in both the book and movie, was a bold, strong and knowledgeable fighter. Here he's turned into nothing more than a complainer and follower to Hazel, who doesn't know anything in war strategy. Pipkin, previously nervous, but extremely loyal, is turned into a Cheerful Child. Kehaar... became unrecognizable. Even Woundwort - the genocidal, slave-driving Big Bad who eats hawks for breakfast and in the book contemplates how some day he hopes to meet a stoat so he can kill it, and remember he's a rabbit - had a period in the series where he tried to STOP THE WAR WITH WATERSHIP DOWN. And OPENLY SHOWED REMORSE for what he'd done. There's more.
While the Character Derailment from the book is fairly rampant, the show suffers a second round at the start of the third season, when the show also had a drastic change in art style and writing. Woundwort is hit the very worst. The season transition happens in the middle of the final battle with Efrafa, leading to a painful moment where Woundwort realizes the error of his ways and tries to stop the war. He's then struck by lightning and the warren collapses. He digs himself out afterwards and declares that he will have become a force of destruction, rather than simply a totalitarian dictator.
Takua was originally an adventurous and brave character who valued duty and helping others (even if he had just met them) above all else, and who would be among the first to volunteer for dangerous quests. In the movie, an easygoing slacker who shoves his duty on others, cowers in the face of danger, and abandons his friend to do his work for him (a friend he tricked into believing it was his work in the first place). It takes the destruction of several villages and seeing his heroes fall for him to realize this is no time for goofing off. Very striking, as prior to the movie's events, he was hailed as a war hero.
Onua, the wisest and most level-headed of the Toa, who always chooses the most effective yet least risky path in every battle (it's written in his official bio), is portrayed here as a slow-minded brute who puts his partners in danger with his actions, and causes the destruction of his hometown, and almost the death of his entire team. Thankfully, this portrayal didn't stick after the movie's saga.
Hahli's supposedly shy (which even the character explorer on the DVD points out) but very friendly, especially towards Jaller, but the movie introduces her as a tough chick who teases Jaller with a much more "sassy" attitude.
Lewa on the other hand was usually portrayed as a juvenile, hyperactive loner with a very sarcastic sense of humor and often held little respect for the other Toa — in the movie, he comes off as a spiritual hippie who actively volunteers to group together with others and seems to be one of the most mature of the Toa.
Vakama goes through considerable change between the second and third movies. At first, he's an incredibly insecure would-be leader who then learns to trust himself, his visions, and not to be afraid to stand up for himself. The following film introduces him as an arrogant, overconfident, reckless oaf who leads his team into a trap despite absolutely everything around him, including his visions, the warning words of his teammates (and a villain he met in the books between the two films) and the threatening signs found all over the ruined city, telling him to act cautious.
Stanley has a minor version: For the first few episodes, Lionel acted like your standard Big Brother Bully. Then, as if overnight, he become a Cool Big Brother.
Andy Panda: In 1944, director Shamus Culhane wanted to try out a new, redesigned Andy that was a total Jerkass instead of a Mickey Mouse-type everyman. The new Andy was hated by Walter Lantz and audiences alike, and so he was dropped after just one cartoon, "The Painter and the Pointer", in which Andy threatened to kill his dog if the dog didn't pose for his painting, and to make sure he didn't move, Andy rigged a shotgun to shoot the dog if he moved from that position.
Quite possibly the character of Sheen from The Adventures of Jimmy Neutron: Boy Genius and its Spin-OffPlanet Sheen. In fact the derailment is due to the Spin-Off. While Sheen could be insensitve and oblivious to to others, it was clear he cared about his friends and family in the Jimmy Neutron series. However, in Planet Sheen, he is utterly indifferent his family and friends. To the point where he doesn't even attempt to repair the damaged rocket to go back to Earth.
Hiroshi Sato from The Legend of Korra. He joined the Equalists because a firebender murdered his wife and also because he believed that it would be the best way to create a safe future for his daughter Asami. Even though Asami sided against him, all signs pointed to Hiroshi just wanting to be able to reconcile with Asami and be a family again. During the finale, Hiroshi even tells Asami that he just wants to be a family again. However, once Asami gets her hands an a mecha tank and starts dismantling the Equalist's airplanes, Hiroshi also gets a mecha tank and after an argument, he says that he has no chance of saving her and tries to kill her.
The Warden of Superjail! losing his sadism after the pilot and gradually becoming more of an outright Man Child (as opposed to Psychopathic Manchild) can be seen as this for some fans, although it's a highly contested example. He also he winds up no longer being someone to be feared, but instead be beat up and easily taken advantage of, and have his obliviousness and naivety cranked Up to Eleven.
The Twins experienced this in season 2's "The Budding of the Warbuxx", due to it putting way too much emphasis on how alien and disgusting they were. Though in general, the outright revelation of them being aliens is taken this way by a portion of the fanbase (as it also coincided with them either being put Out of Focus or having their vulnerability and foolishness emphasized more than their other traits).
While Lemongrab of Adventure Time was always a huge Base Breaker, and even the staff of the show had different opinions of him, after his first apppearance the show seemed to lean towards the idea that he wasn't truly evil, just really messed up. In his story in the third collection of character shorts, he seemingly devours his twin brother. In his next appearance, we see that he's become a tyrant who abuses his now maimed brother, and torments his lemon children when he had previously been portrayed as a loving parent. At the end of the episode, he fully eats his kinder brother, and loses any of the redeeming traits he may have had before. The next episode continues to show him keeping the lemon kingdom in misery, torturing his subjects for fun. He's ultimately torn apart, and then stitched back together, the characters suggesting that now maybe he'll be better. The only thing more sour than Lemongrab is the taste the whole thing left in his fans' mouths.
If Lemongrab isn't all that convincing, Finn might be considered a case. From earlier seasons, Finn starts off as a silly and aggressiveJerk with a Heart of Gold with Chronic Hero Syndrome. Starting near the end of Season 3 and the beginning of Season 4, he ends up having some emotional breakdowns since he can't love Princess Bubblegum anymore, thus loosing all his heroic traits, no longer becoming the Hero Ooo desperately needs. The middle of Season 5 makes him even worse, even before and after the breakup with Flame Princess, the girl he ended up loving after losing PB, suffering a severe case of Took a Level in Jerkass, Love Makes You Dumb and a few What an Idiot moments. He eventually recovers from these 3 traits, however, and considering that he's been entering puberty, it's understandable. The end of Season 5 and the start of Season 6 even shows how this affected Finn as a whole, and its clear it hasn't him done him any good, especially since he lost his arm for a time when trying to meet up with his Disappeared Dad, which REALLY dealt a great deal of emotional damage to him.