For that matter, Mary Jane as well. In One Moment In Time, said deal is retconned so that she is now the one to have agreed, and adding insult to injury, she says, "See, to me, the reason I wanted to get married was to have kids. If that's (children) no longer a part of the equation, then marriage is just a piece of paper."
Then in Superior Spider-Man, Peter's Moment of Weakness that allows Otto to fully erase the remnants of Peter Parker from his mind has been resoundingly criticized by fans of the character as something he would never do, namely allowing an innocent little girl to die to keep Ock from finding out about him.
Black Panther. Poor T'Challa. Opinion is divided about when, exactly (Johns, Hudlin, or Liss) the derailment occurred, but not if. The fact is, in the early 2000s, Panther was treated as one of the smartest men in comics, king of The Plan, and spent his time matching wits with Mephisto, Dr. Doom, Iron Man, etc and coming out on top. Fast forward to the present, where he and Storm have been Strangled by the Red String in the span of basically one story arc, he's lost his kingdom (on account latching onto an uncharacteristic Idiot Ball), and his IQ appears to have dropped about fifty points. His well-documented scientific skills and background have been downplayed if not out-and-out retconned, and he's gone from being "Batman, only better" to "Daredevil, only worse, and have we mentioned lately that he's black?". His villains have gone from major threats like Doom, Magneto, and Mephisto to never before seen Russian mobsters, and Hatemonger. In terms of standing, he's gone from one of the most respected and revered heroes in the Marvel Universe to being talked down to by Luke Cage on a nearly monthly basis. The most recent direction has been hemorrhaging sales as a result, and with Christopher Priest (comics) retired, and a general genre wide lack of interest in minority heroes, it's not clear that anybody has the interest and/or skill to try and save the character.
Wonder Woman. Fans of the intelligent George Perez reboot have dealt with a whole host of derailment. From working in a fast food restaurant to Mike Deodato's "Wonder Thong◊" costume to becoming a bounty hunter to John Byrne trying to turn her into She-Hulk / Babe by increasing the size of her breasts and giving her the standard "wimpy male sidekick" Byrne trademark. During all this, Julia Kapatelis was virtually Put on a Bus. And then there was this. The cries of "RUINED Forever!" were so loud they never went through with it.
Part of the problem is that Wonder Woman is supposed to be a superhero who is also the ideal woman. It's even part of her origin: when the Greek gods breathed life into her they gave her gifts that would make her the perfect woman; the superpowers and equipment came later. Problem is, every writer seems to have a different idea of what is the "ideal woman".
This may also overlap with Values Dissonance; Wonder Woman was created in the 1940s, when ideas about the "ideal woman" were somewhat different than they are today.
Not to mention that Wonder Woman's creator, William Marston, was a femdom adherent and believed in the power of women to 'nourish and nurture through loving discipline' or something of the sort, so his 'ideal woman' had little in common with general '40s conception of an 'ideal woman' and to a modern conception of an 'ideal woman'.
While his ideals and so on were kept intact, Cap's being forced to marry the Idiot Ball probably counts as derailment. The writers forced him to adopt a strategy that had literally zero percent chance of achieving any lasting peace or useful result, because otherwise the storyline wouldn't have had the ending they wanted.
Iron Man. Just a special mention for getting the worst. Going from a hero (albeit occasional jerk) to Der Eisenfuhrer, the armored superhero who doesn't mind throwing his best friends in jail for life. It took over a year and The Movie to undo much of the derailment and there is still a very vocal Hatedom. Unfortunately, most of the people who decided to check out the comics because of the movie quit reading out of disgust, so the Hatedom is still the primary chunk of readers.
Quite a few of those long-time fans will argue that the real Iron Man has been dead for quite some time; the "Tony Stark" in the Marvel Universe right now is just an amalgam between the lame-ass teenage Tony Stark and the Heroes Reborn Tony Stark.
Due to Executive Meddling, pretty much everyone on the pro-reg side got this treatment retroactively as well, because when it was started, the pro- vs. anti- registration debate was supposed to be nuanced, with both sides having good and bad points, and therefore supporting pro-reg side wasn't that out of character for Stark et al. However, the pro-reg side soon was Flanderized by the writers to near-Nazi levels of evil, while its original supporters apparently became complete morons and didn't change their opinions when they realized things were going to crap. Stark got the worst of it, but others, including Hank Pym and Reed Richards, had their characters pretty badly shafted too, especially as the latter was responsible for once giving Congress of the best anti-registration speeches ever delivered.
Except that even then, it would still be completely OOC for Tony. Most of the mainstream dismay over Tony's derailment in Civil War seems to stem from the fact that Tony is a good guy, and while he might not be immune to ever making minor morally ambiguous actions, he would never under any circumstances attack his lifelong friends and take away people's rights. But even if Tony wasn't a frickin' hero and good, moral person; it would still be OOC because Tony hates government control and oversight of superheroes, is extremely suspicious of and unimpressed by proposals to regulate and record sensitive information about his armor technology, tends to be paranoid of people he doesn't know and trust on a personal level due to how many times he, his friends, and his team have been endangered or compromised due to meddling government officials or information falling into the wrong hands, and has sacrificed most of his influence and reputation, a good chunk of his fortune, and on more than one occasion, his mental stability, on keeping The Avengers safe from society and the government by managing, organizing, funding, and legitimizing the team and acting as liaison between it and the federal government, the police, the New York City courts, and various other organizations. If he absolutely had to turn into an immoral supervillain-esque monster, he'd go off the exact opposite side of the deep end into some sort of anarchist rebel.
Tony at least had some means of absolution that vaguely fit the character. Minor characters like Sally Floyd and Ben Urich effectively had their sympathy nuked off the face of the earth - first by telling Captain America that he (representing the ideals of America) was outmoded because he didn't care about the same pop culture as the rest of America (an argument that might have had some traction if Cap hadn't sat there and taken it), and then - keep in mind, these characters are journalists - handed over information to Tony about how one of his underlings tried to trigger a false war with Atlantis to gin up sympathy for the pro-reg movement because "the ends justify the means."
This was a slow process with Batman. From the mid-nineties until early 2006, the cool, gruff, Badass, Goddamned Batman slowly moved from "aloof and driven" to "frickin' jerk". DC eventually fixed this by having him realize how he was acting, and go on a year-long trip around the world with Dick Grayson (the first Robin) and Tim Drake (the then current Robin). This was merely the most recent incarnation of a storyline that's been recurring since the early 90s. Batman would become more aloof than ever before due to some sort of crisis, only to eventually realize that he should be nice to his friends and swear that he would never go down that road again - until next time (See "Prodigal," "No Man's Land," "Batman: Murderer/Fugitive"). The only difference between this storyline and its precursors is that writers seem determined to stick to it for a change, especially Grant Morrison.
Conversely, the switch of Batgirl III/Cassandra Cain from one of the better examples of Rising Above Her Past (raised from birth as an assassin, but horrified enough by her first kill to become a Technical Pacifist) to a StereotypicalCacklingDragon LadyMastermind was abrupt enough to induce whiplash. Nerfing her enough for Robin to force a stalemate was simply adding injury to insult (she is acknowledged as one of the best fighters in the DCU while Tim is likely one of the worst in the Batfamily).The efforts to retcon the whole mess as brainwashing by Deathstroke came off as more than a bit slapdash, and did nothing to explain the improved language skills (what was once a virtually illiterate dyslexic who rarely spoke a sentence more than five words long without the use of pausing, was now Monologuing and knew Navajo code, one of the hardest languages in the world). The latest miniseries about her derailed her character even more. Her improved language skills were taught (she learned to read English, and speak and read Navajo) by Alfred, off screen and she became good with computers by herself. Her deep rooted refusal to kill anyone was removed in order for her to kill her dad and Deathstroke. Her reading of body language (which was used by her to know that Batman was Bruce Wayne) was nerfed in order to let an old man lie to her right in front of her face. And if that wasn't enough, her past was changed from loving her father but escaping from him because her first kill was the first time she saw someone die which made her realize how wrong her life was, into hating her father during her entire life and actually having to watch him kill people right in front of her eyes without her caring at all. And on top of that, the series even managed to derail Rose Wilson, the daughter of Deathstroke.
Many readers felt that Dr. Leslie Thompkins was derailed in the "War Crimes" plotline (following the "War Games" crossover), when it was revealed that she had intentionally withheld care from Stephanie Brown, a.k.a. Spoiler (and one-time Robin) so that she would die in order to teach Batman a lesson. Considering her previous saintly devotion to saving lives, this was a bit stupid and subsequent comics have quietly ignored it, before it was retconned out completely.
The Punisher has had to brush with character derailment ever since he was created. Although his exact quirks and personality vary slightly from writer to writer, in his heyday in the early nineties he was generally portrayed as sincerely wanting to help people and keep them from going through the same things he did, would occasionally question his actions and show mercy if the situation warranted it. Even a few traditional heroes considered him a good man at heart. Other writers would instead portray him as a complete amoral psychopath who didn't care about anything except killing criminals, and never ever questioned his actions.
Garth Ennis seems to have struck a fine balance, portraying him both as a complete amoral psychopath who doesn't care about anything except killing criminals and who never ever questioned his actions, but also as a man who is extremely diligent, going to huge lengths to ensure absolutely no innocents are harmed while he's "working". And on rare occasions, the plight of those around him do get through - just read The Slavers.
Quite possibly the ultimate derailment was Punisher: Purgatory, which set Frank up as an avenging angel. In fact, that was the last Punisher story until Welcome Back Frank, wherein Ennis promptly undid that idiocy.
Attempts to break away from their own title for the stars of Power Pack have led to baffling character derailments, such as when eldest boy Alex stole his brother and sisters' powers and became the laughably-monikered Power Pax, alienating fans for years after the event. Eldest girl Julie Power also changed from a book-reading, highly-articulate redhead (her character was based on her creator, Louise Simonson) in the original comics to a bumbling, dumb blonde actress in the pages of Runaways and a later spin-off series, Loners, where she was notable for magically appearing in one scene without explanation merely to be brutally stabbed so that a male character could be shown to feel guilt at her situation, and having a solo story that somehow convinced several thousand readers to stop buying the book with only two issues to go. Still, the "dumb blonde" part was later shown to be a deliberate facade.
Most fans of Young Justice thought the entire team went through this when they transitioned to Teen Titans. Kon suddenly having Cloning Blues and generally being a too-serious jackass when before he even made jokes about his clone status, and wearing a new costume that consists of jeans and a t-shirt! Impulse suddenly grew grim and studious (and became Kid Flash, abandoning every last trace of his fierce individuality) after Deathstroke kneecapped him and he was forced to endure painful surgery. And Robin was Batman Jr, without a trace of his Deadpan Snarker attitude, his geek hobbies, or the fact he does have a sense of humor. After Kon died in Infinite Crisis, Robin became a total loner obsessed with bringing Superboy back to life at any cost.
Cassie has become very ill tempered and cold towards her teammates after One Year Later. Her character has downgraded into a self-righteous, holier-than-thou Ice Queen who is obsessed with bringing Conner back and wants to change the team back into Young Justice. Pre-One Year Later, the common criticism about her involved Johns having watered down her geekiness and outgoing nature to turn her into someone who only seemed to exist to be Kon's love interest and not have much else to her.
Sean McKeever seemed to have noticed Cassie's change, and attempted an Author's Saving Throw by blaming it on accepting powers from Ares. This negated the powers Zeus gave her and would have corrupted her completely, were her will not strong enough to resist him. Instead, she just turned into a bitch. Although the following writers, especially Felicia Henderson, promptly derailed Cassie back into a raging shrew.
While on the subject of Young Justice: Inertia (Thaddeus Thawne), Impulse's Evil Twin. Originally, he was a rebellious teenager who secretly resented Impulse for having something he never had: a family. In fact, that eventually drove him to abandon the people who were using him to try to make his own way in the world. Then he became a generically evil and sadistic version of Kid Flash who engineered his good counterpart's death. Finally, Johns decided to have the character take on the title "Kid Zoom", kill a child, and depower the actual Zoom. Of course, the end of the story showed that Thad's actions would have consequence, and he wound up dead as a result of pissing off the other Flash Rogues for his crimes.
When the second Aquagirl (Lorena) was introduced in the pages of Aquaman, she was a resourceful Plucky Girl with brains who learned to adapt fine to the ocean after she lost her entire family and all of her friends. Once she became a Titan? She became derailed into a horny and mouthy Latina stereotype, trying to play homewrecker to Blue Beetle and Traci 13, while serving little other purpose than to hit on boys, argue with Bombshell, or get seethed at by a jealous Cassie.
Some argue that Geoff Johns' changes to Raven have stained her character reputation irreparably. These include having her reborn as a teenage girl (after she spent time in limbo as a golden Spirit Advisor) who only occasionally retained her original speech patterns and personality (while the rest of the time she had a snarky and broody attitude like her animated incarnation), hooking up with Beast Boy, and saddling her back with the position of being the Distressed Damsel that the team must rescue. Later writers like Judd Winick only made these changes even more jarring.
On the opposite end, Beast Boy fans have become upset that after having received development in his own mini-series, he started on a gradual decay back to being the team goofball to the point where even his own best friend and younger team members were depicted as talking down to him and considering him to be a joke. The same Beast Boy who became team leader of the Titans and was considered capable (before executive meddling hit, combined with Geoff Johns deciding the team had "too many adults").
Despite having moved on from a troubled past which included alcoholism, rampant womanizing and generally irresponsible behavior and evolving into a loving, responsible father and boyfriend under Kevin Smith's pen, Winick wrote Oliver Queen back into the clueless, womanizing, limousine-liberal stereotype many comic fans wrongly saw him as.
It is also worth noting that - despite Winick's portrayal of Queen as an unrepentant ladies' man - Oliver Queen never cheated on long-term girlfriend Dinah Lance (aka The Black Canary) before Judd Winick started writing the character. He did father a child with Dragon Lady Shado, but that was the result of Shado raping him while he was drugged. In Winick's first story arc, Oliver Queen had a one-night stand with the niece of fellow superhero Black Lightning and later tried to lie about the affair to Dinah Lance. Interestingly enough, the two had never been shown to have officially reestablished themselves as boyfriend/girlfriend until Winick chose to break them apart.
To the astonishment of fans everywhere, the title has actually managed to get worse since Winick left, highlights of new writer Kreisberg's work including Ollie going off the rails about how useless nonlethal crimefighting is (despite having dealt with the whole killing thing decades earlier in what's probably his single most famous story and subsequent run), and Dinah's nurturing hero-focused childhood amongst her JSA 'uncles' being retconned into a Wangsty life of ignorant normality until the day she accidentally permanently deafened a friend with her emerging superpower. In order to mirror her incompetent adult use of said superpower, wherein Kreisberg caused her to deafen an innocent bystander in a fight so he could give her a new supervillain. Reactions have been fairly uniform.
Jefferson Pierce (Black Lightning) went from being a Technical Pacifist of such strong ethical fiber that he retired from superheroics when he thought he couldn't use his powers safely into a man who could easily strike down the corporate raider indirectly responsible for the death of his niece.
After taking flack from numerous fans as well as Black Lightning creator Tony Isabella, Winick retconned that last one in Outsiders #45-47, where Jefferson turned himself in for the murder. More, he was revealed to be innocent of the crime, thanks to super-assassin Deathstroke just happening to be in the area, just happening to guess what Jefferson's internal conflict was and just happening to have the perfect Deus ex Machina to kill the man and have it look like death by electrocution.
In the original Inodoro Pereyra comics, Eulogia was beautiful and nice. During the 70s, she became fat and cranky.
Agatha went from being Gaturro's girlfriend to his unattainable love interest.
In the original stories of Paperinik (aka Phantomias), Paperinik was the Chaotic Neutral alter-ego that Donald Duck used for punishing those who had wronged him, mostly his own relatives. He was a shameless outlaw who was hunted down by the police and never had a motive to help others, unless he'd get a large reward. (Back then his relatives were HUGE JerkAsses. Scrooge was always breathing down his neck, pushing him and so on. Daisy was being a Tsundere. Gladstone was acting like Gladstone. They mellowed out later.) In later versions he was turned into a Lawful Good superhero who fights crime and unconditionally helps the authorities. This was a derailment for the better, out of which we got Paperinik New Adventures.
Blue Beetle (Ted Kord) and Booster Gold over the twenty years that these two have been friends and thought of as a duo they have been so MASSIVELY derailed that they practically switched personalities. Booster was originally a Fish out of Temporal WaterMr. Vice Guy who screwed up once in a while but learned from it, while Ted had Jumped at the Call and was basically Batman with a sense of humor. By the time of Super Buddies, Ted was now a I Just Want to Be Normal slacker who was letting himself go and was now The Straight Man. Booster got it worse as he seemed to have permanent ownership of the Idiot Ball and was the one who wanted to have fun all the time and was such a screw up that they coined the term "Boostered" after he accidentally sent the team to hell. Needless to say those comics make many fans flinch.
The Flanderization that took place before that was retconned into Obfuscating Stupidity. Beetle was killed in Countdown to Infinite Crisis and Booster has a new series where he's treated more seriously. Some fans, however, greatly enjoyed the Superbuddies and were greatly disturbed by the Character Derailment of characters such as Max Lord, Fire, Mary Marvel, and a number of the others.
Maxwell Lord was never the "nice guy" on the team, he was certainly a Jerk with a Heart of Gold but was it ever a question about the heart of gold part? No. He even thought and believed he was doing the right thing most of the time and showed genuine concern for his team, back when he headed off the group. He even got into an argument with Martian Manhunter about how they needed to get "big guns" on the team to make sure no one would get offed. It's not like it was all dialogue, some of this was in thought bubbles, so that means he thought he was a hero, not a villain. So is there any reason as to why he should suddenly become a cliched villain two steps above strapping Penelope Pureheart to a train track? Or any reason why he would blow Blue Beetle's head off, or claim he was lying all those times (even to himself apparently!) he said he was a good guy? The writers actually admit they know they derailed him but don't care because they needed a villain... the writers knew they were doing something that flew in the face of his prior characterizing and did it anyway! Wow. Just wow.
There are two explanations for this, a fan one and an "official one". The fan one is that Max was still under the influence of Kilgre, even when he became fully human again. The "official" one is that Superboy-Prime's punching of the Source Wall retroactively influenced Max; while he was sincere before, during his many surgeries and procedures to become human, he gained a hatred of superheroes, presumably because the community at large was responsible for him being a cyborg in the first place.
An issue of Justice League Generation Lost showed that Max only went off the deep end after Coast City (Along with his mother) was totalled by Mongul.
The Amazons Attack miniseries afflicts Wonder Woman and the entire Amazon people with character derailment by way of Idiot Plot.
Infinite Crisis saw Superboy Prime move from one of the guys who saved all of reality to a Knight Templar obsessed with finding the perfect Earth. Later stories moved him all the way into Omnicidal Maniac territory as he crushes entire planets because he happens to think they're lame.
Just think of it this way, the only appearances of Prime are in Crisis On Infinite Earths, Infinite Crisis, Green Lantern (especially the "Sinestro Corps War" arc), and Final Crisis: Legion of 3 Worlds. Prime's appearance in Countdown to Final Crisis never happened, just like the rest of ''Countdown''. This at least gives him a relatively stable character transition: Multiversal hero to Knight Templar to The Dog Bites Back to frustrated semi-Omnicidal Maniac who wants to destroy Superman's legacy and show he is superior.
Okay, I know it's only a story about a possible future and, arguably, not in the main continuity but Bruce Banner in the climax to the Wolverine story Old Man Logan who is revealed to have become a sociopathic, redneck cannibal who is now head of a family of in-bred Hulklings. For added squick it's revealed that this family resulted from Bruce breeding with She-Hulk, his cousin - "The only woman out there who could take the damn pace" (emphasis as original). There's also a strong implication this was not consensual. Ugh, just, ugh. Compare this with Banner's actions in Incredible Hulk 602 which came out the same week to understand exactly what's wrong with this picture. Naturally this is another Mark Millar special.
Apparently in this universe Banner's long years of exposure to gamma radiation finally took their toll on his two personalities. So for this reason the Banner persona disappeared, left with nothing but an insane Hulk.
If one considers Mark Millar's other major work involving the Hulk, The Ultimates, also cast the Hulk that way, and that continuity was specifically created to show case "new interpretations" of the characters, i.e., the writer's interpretations, one gets the impression that Millar isn't a Hulk fan.
Ed Brubaker did this to Black Tarantula in his Daredevil run. Black Tarantula was created as an elegant boss of a criminal empire who put himself above everybody else, is an heir to ancient power and Implacable Man who goes toe to toe with Humongous Mecha. Brubaker completely changed his character, gave him father issues, turned his empire into an ordinary gang and weakened him without giving any reason. Some say that he's gone from a badass sinister crime boss to a low level stereotypical thug, while others point out that in certain Spider-Man stories he was getting really close to Villain Sue territory (he defeated Spidey twice and spared his life because he didn't see him as a real threat).
Countdown to Final Crisis does this to Mary Marvel in a very annoying fashion. If not her character arc of accepting Black Adam's powers, going a little nuts and joining up with Eclipso before learning the error of her ways and helping to rescue the Greek gods, then it certainly counts when she, after this long storyline of turning evil and being redeemed, joins up with Darkseid AGAIN!
Matt Fraction has done this several times. Maria Hill from Badass NormalAction Girl turned into The Chick for Iron Man? Check. Weapon Omega, guy who was always afraid of using his powers and never wanted to hurt anybody, becoming a psycho who likes killing people? Check. Ares, master strategist and badass god of war turned into stupid brute who says only Aye? Check. Pixie drained of all her character and transformed into Fetish Fuel Station Attendant for all loli fans? Check. Emma Frost flanderized into Flat Character who is only defined by calling people "darling" (and there is already large group considering her being derailed in at last five different ways. Fraction's version don't match any portrayal of her)? Do a number of fans think Matt Fraction just doesn't understand the characters he's working with? Hell yeah. Recently, he managed to screw up Tony Stark's characterization and entire history in one issue, including things established by Fraction himself.
In the new Defenders series, we now have Doctor Strange, sexual predator, and Betty Ross, slightly petulant adrenaline junkie who keeps forgetting she's invulnerable.
Hawkeye is a pretty blatant example. After spending years on the Avengers as the biggest mouthpiece of "Avengers don't kill", he died in Avengers Disassembled. He later came back during the whole House of M mess, tried to kill Wanda, spent a few years mostly normal and then, in Matt Fraction's run, started casually killing people in his own series and mentally congratulating a teenaged girl on doing the same.
Brian Michael Bendis has a tendency to be horribly tone-deaf on some characters. That Photoshop that used to be the page image? That was originally Dr. Doom — ruler of Latveria, world-class evil genius, and refined bastard — telling Ms. Marvel to "shut [her] cow-mouth" or else he'll stop her "whore's heart". And what did Ms. Marvel, veteran superhero and military brass, think about this childish outburst? "That hurt my feelings." Carol, when the supervillain who matches wits with Mr. Fantastic breaks down and calls you imbecilic gutter insults, you don't feel bad. You take a picture so it can last forever.
Try reading Alpha Flight some time. From Byrne to Mantlo. 'nuff said.
Owen Mercer, the second Captain Boomerang, was never a completely good person. At his best, he reached Jerk with a Heart of Gold status. But he was trying to move away from the family legacy, and trying to do good, and won himself friends like Nightwing and Supergirl in the process, then vanished from the comics for awhile. Then in Blackest Night, he pops up as an unhinged psycho feeding children to his zombie father, and is promptly killed for it. And then the white rings resurrect his father in his place.
Hank Hall in DC's Hawk and Dove was an impulsive Jerk with a Heart of Gold in the Kesels' run, having been fleshed out significantly from the original Steve Ditko incarnation and his appearances in the original Teen Titans. However, once the identity of Monarch was leaked as Captain Atom in an advance spoiler for DC's mini "Armageddon 2001" (though there had been foreshadowing that this was the case to begin with), editorial had to scramble and find a new character to be Monarch to retain the "surprise" ending. Unfortunately, they picked the one character that was blatantly shown NOT to be Monarch and a perplexing plot twist followed, derailing Hank into a murderous extremist and suddenly advanced enough in intelligence and powers to control time (with yet another villainous name change as Extant). He then lingered on as a sort of Villain Sue until he was killed off in the pages of JSA and then later brought back in Blackest Night, though it remains to be seen how his characterization will fare.
This trope came down on Rotor hard in Sonic the Hedgehog issues #215-#216; normally, he's depicted as a shy, introverted Hollywood Geek who prefers to stay in the background and only fights when he needs to, but these two issues, in order to pin him as the faux traitor in Ian's ongoing story arc with Silver, twisted him into a bored glory-hound with a self-esteem issue who nearly killed his friends in a previous and ill-conceived attempt to play the hero. Even worse, despite being touted as one of the greatest minds of Mobius for most of the comic, these issues made him come off as dumber than average, spouting inaccurate Techno Babble and pushing the development of his projects off to NICOLE instead of doing it himself.
J Jonah Jameson has always been a jackass (whether there was anything morethan that depended on the writer) but he also tried as hard as possible to reveal the truth about any Villain with Good Publicity. Recent stories have his hate for Spider-Man go from irrational to certifiably insane and him becoming a vocal supporter of Norman Osborn, a man who treatened to kill his family in the past.
Canonically, Death's Head II is supposed to be the same character as Death's Head. However, many fans felt he was not the same character — he was not written by the original writer, exhibited none of the mannerisms and personality quirks of the original, and ended up as a generic Darker and EdgierBoring Invincible Hero. It got so bad that Death's Head's original creators Simon Furman and Geoff Senior wrote and drew What If... #54 just to show their take on what should've happened instead.
Buffy the Vampire Slayer. When Twilight, the villain of the Buffy the Vampire Slayer: Season 8 comic, is revealed to actually be Angel this represents a major case of character derailment. We're supposed to believe that someone who'd spent five seasons learning that the battle against evil is probably hopeless, but you fight it anyway has suddenly decided to abandon the world to destruction in order to bring about his own happiness? Right.
It's all the worse when you consider that as far back as the episode "I Will Remember You" of Angel, Angel was willing to sacrifice his happiness with Buffy in order to be able to fight the good fight against evil. In the entire Buffverse arc since they, nothing has been seen to contradict this, and suddenly he could give a flying fuck... literally.
Deconstructed in Maus. The story continually compares the generous, brave, resourceful Vladek Spiegelman who survives the Holocaust to his present self, who has inexplicably devolved into a cranky pain that makes life miserable for everyone. Part of this (non-fictional) account deals with the author's issues and incredulousness at the difference between his father's behavior then and now. The story also rejects his Freudian Excuse of behaving the way he does, noting that other Holocaust survivors didn't become the bitter shell he is now. It should be noted that the portrayal of Vladek as he was during the Holocaust could be a case of an Unreliable Narrator; the only source of information we have is Vladek himself, and attention is drawn several times to the fact that Vladek's memories sometimes contradict themselves or other eyewitness accounts. Also, it is debatable whether Vladek's post-Holocaust misanthropy is truly a derailment from what he was during it; there is never any question that he loved Anja (his wife during the Holocaust, who committed suicide a few decades after it was over) far more than Mala (his wife at the time of writing), so it makes sense that he would be far more compassionate towards Anja. Old Vladek also shows resourcefulness, but because it is unnecessary in his time it comes across as simple miserliness. There are also hints from Vladek's possibly rose-tinted memories of himself that suggest he might have been somewhat domineering and lacking in sympathy even then.
The '2010 'Batman Beyond'' mini series does this to almost every single character in varying degrees, from Terry forgetting he has a girlfriend and picking up the Idiot Ball - apparently losing about 3 years of experience in the process - to Bruce suddenly deciding that Terry just isn't good enough anymore and constructing bat robots to replace him. This is without getting into all of the logic and continuity fail that is eeeeverywhere, or characterization like Dick's... which probably classifies as character derailment even taking into account the fact that he spent 30-40 years being bitter and alone, because his characterization is so far from the DCAU starting point (whose characterization is different from his comics counterpart in a fairly substantial way) as to be baffling.
Not a single mention yet of David Reid? Introduced to the Justice Society of America as the great-grandson of FDR, Reid joined the team as an earnest but dedicated rookie with a solid respect for the team of veteran superheroes. Then along comes Gog, who transforms Reid into Magog after his brief brush with death. The transformation influences Reid into acting brashly and recklessly, but once he sees what Gog's really about he turns on his master, even severing Gog's head in the climax. In the aftermath, he apologizes to Alan Scott and is seen without his trademark eye scar◊, indicating he's earned a fresh start. So what happens after that? He's quickly flanderized into a caricature of his Kingdom Come counterpart in every appearance other than his brief miniseries and ultimately killed off by Maxwell Lord in a really ugly death scene.
The Kid Amazo storyline also derails the whole League. Flash jumps at the chance to kill a cyborg kid who hasn't done anything wrong yet, J'onn doesn't talk remotely like J'onn, the group as a whole is a bickering leaderless mess...
On the X-Men page linked above, it's been noted that Warren Ellis's approach when writing for mainstream comics lately has been "take a look at past few issues featuring character, extrapolate from there as baseline behavior." If you want any further proof that's the case, then behold as CaptainAmericacondones torture.
The Mega Man comic had Mega Man himself. In the first four issues Megaman grows proud, power hungry, and then into zen mode as he fights the Robot Masters. In the Megaman 2 story arc, he then grows more and more destructive and merciless due to evil programming. This wouldn't be so bad in and of itself if not for the short gap between the stories and the fact that his core personality, the noble, childlike hero, hadn't really been developed or observed yet.
Avengers Arena: Fans of the characters before Arena find many of their actions within to fit this bill. Primarily, it's turning characters with deeper complexities into shallow caricatures of their former selves or have reverted to old characterizations they had moved passed.
Mettle is defined entirely by his relationship to Hazmat. Worse, he engages in an action which goes against the sacrifice he makes in the "Final Exam" arc of Avengers Academy.
Juston's prior characterization is jettisoned entirely so he can be defined by depression and revenge. This characterization is not without reason, but there is nothing else about his character that shows up.
Nico, a mothering character, never mentions the people whom she cares for, even while she is dying.
Chase acts more like he did in the last arc of Brian K. Vaughn's run rather than showing the development and maturity he gained under Joss Whedon.
Cammi is basically a new character, but the progression is logical from where she left off at least.
Reptil is supposed to be the leader of Avengers Academy and has properly lead them on several occasions. In Arena, this is not in evidence and in some scenes he seems to cede leadership duties to Hazmat.
X-23 has a major out of character moment even within the context of Arena itself when she attacks Apex head on, despite knowing the latter is in control of a Sentinel. This is after the series firmly establishes that Laura is constantly determining multiple means of killing everyone around her, choosing the best course of action while discarding those with undue risk.
Fire Princess Ursa, mother of Zuko and Azula, and the single biggest hanging plot thread of Avatar: The Last Airbender. Her fate would finally be revealed years later in the sequel comic The Search—but the explanation of her disappearance cast her in a far more negative light than what most fans expected. To start off, she's revealed to have written a letter claiming that Zuko wasn't Ozai's son, but actually her lover Ikem's son just to see prove that Ozai was reading her letters. He was, but saw through the lie anyway, but vowed to treat Zuko as though he wasn't his son to punish Ursa. Meaning that part of Zuko's terrible childhood came about because of Ursa trying to spite Ozai. More importantly, the trauma of being Ozai's wife became too much for her, so she asked the Mother of Faces to remove her memories of her life as a Fire noble, even though it would mean forgetting her children and leaving them in the hands of Ozai. Despite that, she allowed herself to keep her memories of Ikem, and ran off to start a new family with him. There is heavy debate on whether this is Character Derailment. Some fans argue that very little was shown of Ursa in the original series, and most of what was seen was shown through Zuko's eyes, leaving some fans to wonder if Ursa was really the perfect mother Zuko perceived her as. While some consider the popular fan perception of Ursa to be just fanon, detractors counter that what was shown of Ursa, particularly her last words to Zuko ("Never forget who you are" and "Everything I've done, I've done for you" which would be a flat out lie given the revelations of the comic) paint her as a far stronger and caring mother than what the comic gave us. They also point out that the people who wrote the original Ursa flashbacks weren't the same people who worked on the comic. Ursa herself before and after regaining her memories says what she did was a terrible thing for a mother to do.