Rosie Webster is currently a superficial ditz who can't even cook without burning the kitchen down. The writers clearly forgot that she went to a posh private school for the gifted. And about her deep and loving relationship with Craig Harris in the past, because now she's the biggest whore on the street. There was a time when she was the responsible sister who had to look out for Sophie.
David Platt went from a troubled teenager due to an overprotective mother to being a juvenile delinquent who smashes up the street, frames someone for robbery and attempts rape. Then jumping back to Jerk with a Heart of Gold as an adult, wherein he is still rude, and selfish, but overall a moral person.
Although he's recently devolved (and gotten worse) than his original characterisation and is currently barely a step away from being a total sociopath.
Molly Dobbs was a capable and good natured woman who valued trust and honesty in a relationship above anything else. Now she's had a full blown affair with a married man who happened to be her husband's best friend. And when his wife got cancer she couldn't understand why he was finishing with her.
Sunita Alahan. She was once one of the most moral people in the show. Suddenly, she's having an affair, wearing skimpy outfits and being plain nasty to everyone for no good reason.
Alex went back to being a slacker without ambition despite readmitting herself back to Degrassi to get higher marks and pursue a health profession the season before. Her lazy, judgmental, and unreliable attitude caused her second breakup with Paige as well as her character being put on a Long Bus Trip to Ajax where she has "options".
Ashley wanted so badly not to be seen as Craigs' sidekick but a musician in her own light, she ended up accompanying Craig on a European Tour after Spring Break. This is made worse and confusing because she wasn't there to take her final exams so it's unknown if she graduated high school. And considering that Craig now lives in Hollywood further pursuing his music career, it is unknown if Ashley even got hers off the ground during her time in Europe with Craig.
Not to mention her behavior beforehand, which consisted of erasing then-boyfriend Jimmy's rap track from a demo they were collaborating on without his knowledge or consent—because she was jealous that he had upstaged her.
In some cases, this started long before season 7. For example, Ellie, from her introduction in season 2 until season 4, was a sarcasticgoth/punk girl who put everyone else before herself no matter how much it hurt her to do so (acting as Marco's beard, taking care of her alcoholic mother, etc.), then pretended to be fine when she wasn't and then vented her emotions through self-destructive behaviour. She also hated Craig, a lot, and, despite playing a few instruments, was determined to follow her passion for journalism. In season 5, her wardrobe was radically changed, and she became a clone of Ashley- much bubblier than before, petty enough to throw a drumstick at her romantic rival, and willing (though she didn't actually end up doing so) to follow Craig, who she had inexplicably fallen for despite never resolving her relationship with Sean, to Vancouver to start a music career. By her last appearance in season 8, she was practically unrecognizable, both physically and in terms of personality.
The iCarly episode "I Date A Bad Boy" derails the titular bad boy in record time. At the start of the episode, he is a punk kid criminal who seduces Carly. Halfway through the episode and without any warning he is derailed with the introduction of his shameful (to the main characters, but not to him) collection of beanie babies and is immediately reduced to an obsessed man child.
Double The Fist: Why is Steve trying to take over the world? Steve who was previously willing to give his life to save EVERYONE on the planet?
Quinn Mallory on Sliders went from being a likable boy genius in the first two seasons to a cardboard action hero who had a different girl each week from Season 3 onward. He also didn't seem particularly concerned that Arturo (a father figure) was killed in front of him or Wade (his best friend) had been kidnapped by the Kromaggs.
The arc dealing with Olivia's discovery of her half brother on Law & Order: Special Victims Unit, an until-then realistic show with realistically sensible characters, reduced Olivia to possessing all the common sense of your average soap opera character. She and Cragen even note later that she was Not Herself.
A worse derailment happened much earlier. In the very first episode, Olivia's mother appears (played by Elizabeth Ashley), and the mother and daughter seem to have a loving, complex relationship, with Olivia's mother being more at peace with the facts of Olivia's conception than Olivia is. Retconning Olivia's mother into an alcoholic, who emotionally abused Olivia because she reminded her of her rape, is a lazy way the writers have capitulated to people's assumptions, with a 2-dimensional character, probably just to provide a source of easily tapped melodrama without having to work at real story-telling.
Amy in Buffy the Vampire Slayer went from being an everyday witch to being a cartoonish bad girl for no reason at all. Shortly afterward, Willow inexplicably became addicted to magic (an effect not been shown in the series before) and started doing morally dubious things like wiping her girlfriend's memory of an argument they had had.
It's implied that Amy's delinquency was part madness from having spent so long as a rat (really?), and part culture shock from being four years behind everyone else. Doesn't make it any less abrupt or clumsy, though.
In high school Buffy was able to turn her graduation class into a commando squad to take down the invincible Big Bad. Four years later she goes off about one girl who the First Evil had Driven to Suicide then chews out the rest of the potential Slayers because they don't have her powers and experience, thus are not worthy.
Giles leaves Buffy in season 6 and goes back to England so that she can learn to cope on her own and become more responsible. When he returns full-time in season 7 he is annoyed that Buffy doesn't listen to his advice to the degree he actually plots against her - luring her away for the night so Robin Wood can try to kill Spike and then sending Spike and Andrew away so they can't defend Buffy when he leads a mutiny against her. He also completely out of character criticises the gang for having a few laughs at one funny situation for no good reason at all.
All of these things are completely in character for Giles. He is shown multiple times in Season 5 to be willing to kill innocents in order to protect the world. In Season 7 he returns and finds Buffy protecting Spike, even after it's proved he's become a killer again thanks to the First. The reason he plots to get rid of him is to protect all of the other heroes fighting the first and to protect Buffy. He believed Spike could turn on her at any point and wanted the loose end tied up.
The entire Scooby Gang gets it at the end of season 7:
Dawn, despite being a bit naive, never showed any sign of being the sort of person to abandon someone because it was convenient, but right before the Series Finale she kicks Buffy out of Buffy's house.
The rest of the Scooby Gang, except Buffy, proceed to let her. Despite the fact that Buffy saved their lives numerous times, and the fact they cared about her so much that 2 years ago they had brought her back from the dead.
Buffy, instead of protesting the actual kicking her out, complains about how they won't follow her on a poorly thought out plan to get something from the place they had just been defeated. Then kicks someone else out of their house.
However, all of the Scooby Gang derailments are in order to facilitate Buffy's romantic relationship with Spike - Buffy started acting nastier to stop the others from turning completely unsympathetic when they turned against her, Giles plotted against Spike so Buffy could defend him, and the others turned away from Buffy and kicked her out in order for the writers to show her leaning on Spike and depending on him, with him as her only supporter. Your Mileage May Vary on whether this slightly excuses the derailment or makes it ten times worse.
Possibly. There is a simpler explanation - Buffy's poor leadership technique had been tolerated up until then, but near the end of Season 7 virtually everyone had had enough of her superiority complex. Buffy had continually stated the belief that being Slayer made her a) the leader and b) worth listening to, while adding a dash of 'nobody else gets to have an opinion'. Willow and Xander turned against her when it became obvious Buffy's stubborness was going to get people killed. Giles was very quick to see this, but was also very alarmed at her blindness towards anything involving Spike (see above).
These examples actually show that it's Buffy, not anyone else, who turns into a worse and worse person as the season progresses. Her own friends end up tossing her out of the door, because her sheer arrogance is going to get a lot of people killed. Buffy is always the common denominator in these Scooby 'out of character' moments, which is actually an example of the writers piling on the angst and imperfections in the character.
Angel had a lot of derailment complaints at the end, when Angel went after the Circle of The Black Thorn. Fans complained he was too preoccupied with sticking it to Wolfram and Hart to recall his original mission. Not unlike Season 2, really.
In Casados Con Hijos (the Spanish version of Married... with Children), Florencia Peña's character matches the original portrayal of Kelly.
In an odd example where adding positive traits constitutes Character Derailment, Peggy Bundy, during the mercifully short time when Seven appeared on the show. Whereas before Peg didn't give a rat's ass about her children, and cheerfully left them to starve while she scarfed down Bon Bons (and occasionally went out to eat with the money she stole from Al), Peg somehow acts as a responsible parent to that little rat Seven, feeding him and taking him to the doctor. Fortunately, once Seven disappeared, Peg went back to the lazy, self-centered, nagging shrew and negligent mother the fans knew and loved.
In Family Matters, one of Extraverted Nerd Steve Urkel's redeeming traits was originally that he was a personification of the aesop "just Be Yourself." The original appearance of his alter-ego Stefan Urquelle was merely a vehicle for Anvilicious preaching of this aesop. Unfortunately, then someone on the creative team decided that Stefan should become a regular part of Urkel's bag of Mad Scientist tricks; not only did this result in a Broken Aesop, but the entire point of his character was lost.
Early episodes of Everybody Loves Raymond had Ray simply play the kind-hearted guy who wanted to please people, while Debra was the more "real" of the two (that is, not afraid to step on a few toes). By the time the series ended, Ray was out-and-out spineless, and Debra would get onto him for the smallest thing.
Not to mention that in the earlier seasons, Ray was a witty Deadpan Snarker, and was quite clever (or at least possessed average intelligence), and his newspaper column actually won a national award. About halfway through the series, however, his IQ appeared to suddenly plummet. A character who had been the Only Sane Man at the beginning of the series suddenly became the Butt Monkey. This was seemingly done to make his wife Debra appear smarter so that the show could use the Parenting the Husband trope.
Strangely enough, it seems to have gone the other with Ray's brother Robert. Early in the show, he was incredibly socially awkward, still living with his parents, and actually came off as mildly autistic at times. Later in the series, he seems to have taken on Ray's Deadpan Snarker characteristics, acting as the Only Sane Man. Perhaps it's proximity to Ray's parents?
The King of Queens went through something similar. In the earlier seasons, Doug wouldn't hesitate to call on Carrie for her underhanded antics, and she would learn her lesson. As the show went on and slid down the cynical end of the scale, Doug became a gutless complainer, and Carrie's anger problems and superiority complex got taken Up to Eleven
Doug even stumbled into a self-help group of battered husbands once and had a realization about Carrie's behavior.
In the pilot and first few seasons of Three's Company, Chrissy was an innocent but reasonably savvy and intelligent young woman. By the third or fourth season, she had become the living stereotype of the Dumb Blonde—completely naive and illogical, with asymmetrical platinum ponytails.
Dwayne Wayne from A Different World went from being a nerd with a crush on Denise Huxtable, to a super hip and cool teacher other students looked up to within a short period of time. Likewise with Whitley Gilbert, who was the stuck-up, snobbish, rich southern princess, and then turned into a compassionate teacher and love interest of Dwayne. This is likely due to the fact that Lisa Bonet left a huge hole in the show when she left (the show was originally built around her character), so the writers was forced to make Whitley do a unconvincing Heel-Face Turn in a short amount of time.
Both characters evolutions took place over several seasons. That's realistic and hardly a short amount of time. It's FREDDY's derailment that makes no sense—over ONE season break, she goes from being a rebellious, flaky (yet, somehow, brilliant student) hippie-esque love child, to a perfectly coiffed, suit-wearing, pre-law student, fawning and gushing over Ron, who she couldn't stand. Similarly, Ron goes from adoring Kim and hating Freddie to lusting after her.
Seriously, with Whitely and Dwayne, those changes happened over the course of 6 seasons. That's Character Development not derailment.
Speaking of Denise Huxtable, on The Cosby Show, she went from spirited, spunky, funky, intelligent, and independent, to flaky, clueless, free-loading, moron who dropped out of school but didn't bother to look for a job because "it's my summer vacation". Which itself may have been Reality Subtext given Bonet's appearance in Angel Heart, which thoroughly infuriated the show's star, Bill Cosby. (Not that it justifies doing it to the character)
Adam's decay from gentle, caring, and "all about" Joan, to just another horny teenage boy, in the second season of Joan of Arcadia. Actually, "decay" would suggest there was some time between "gentle, caring" and "horny". The Reveal was literally dropped in out of nowhere in the middle of an episode.
More of a concept derailment, but you can point to the exact episode where it was decided that the Prophets in Star Trek: Deep Space Nine went from being uncaring aliens who happened to be worshipped by the Bajorans to being 'of Bajor' and caring deeply, in their own inscrutable way, about what happened.
A character example would be Dr Bashir. He was introduced as young, arrogant, brash and insufferable who gently mellowed out through lessons and some hard knocks into a very confident, but also likable and maturing character, as the series developed. Then, towards the end of series 5, he was revealed to have been genetically engineered as a child and had spent years hiding genius intellect, Improbable Aiming Skills, the ability to use Ludicrous Precision and became a classic example of Suddenly Always Knew That. He was pushed so far into Insufferable Genius territory that his original characterisation at the start of the show seemed mild and humble by comparison. His actor hated it as much as the fans did. (Fortunately, Siddig at least ended the Spock Speak by deliberately flatlining them until the writers gave up.)
Odo. He went from someone who abhorred injustice and the Occupation, argued passionately against destroying a proto-universe that threatened the station, and even got narky about cutting flowers for bouquets, to having a future version who wiped 3,000 people out of existence to save Kira and give his past self a chance with her. Then the writers decided that the only reason that he didn't join the rest of the Changelings—rulers of the Dominion who happily practiced genocide—was because of Kira. You could do some Alternative Character Interpretation with the fact that the people who insisted this the loudest were other Changelings, but Word of God says no, that's really the reason. Never mind all the important relationships he has with Sisko, Quark, and Garak, or his previously-established sensenote albeit an unorthodox one of justice and morality.
The Office seems to be coming close to this trope in the fourth season, making previously sympathetic characters much less likable. Ryan's change from average guy to suave douchebag is at least a somewhat believable Character Development, resulting from his huge promotion. Although Jan was never nice before, she was about as pleasant as you could expect anyone having to deal with Michael Scott to be. After being fired, she transforms into a total bitch. Toby was originally just a guy who got picked on excessively by Michael, but it's strongly implied now that he's jealous of Jim's relationship with Pam, and passive-aggressively taking it out on Jim.
On The Big Bang Theory Leonard started out as a likeable, Dogged Nice Guy, who had a crush on Penny. By season 3, he's a snarky, short tempered, horn dog. It's like the writers forgot how to properly write his character. In his defense, spending that much time around Howard and Sheldon probably played a factor. Thankfully, the recent seasons seemed to have reversed this for the most part.
Recurring character Stuart, the comic book store owner, appears to have his personality do a reversal since his introduction. In his early appearances he was a lot more confident and was generally well-functioning, albeit slightly socially awkward. Stuart has since become a severly depressed loner in which recurring jokes are centered around his depression, suicidal thoughts, and collection of antidepressants. New viewers probably wouldn't believe he had once impressed Penny into going on two dates (that went well enough that he nearly scored both times).
Raj goes through several changes over the series. Initially, he's portrayed as very shy around women, needing alcohol to give him the confidence to talk to them. This later evolves into what seems like a rigid medical condition rather than a lack of confidence - there are a few situations where he suddenly stops mid-sentence when the alcohol wears off or when a woman enters the room. He then goes through another personality shift, going from shy and nerdy to feminine and camp.
JD on Scrubs. He starts the series as a nervous, somewhat needy young doctor who also displays genuine wit, intelligence, charm and sensitivity. At this point, he's something of a second Butt Monkey, has had his emotional neediness become a classic example of Flanderization and has, quite frankly, degenerated into something of an idiot. An inability to locate Iraq on a map or explain the difference between a Senator and a Congressman are the two most flagrant examples of that, but are far from the only ones.
The sixth season wreaks havoc with other established characters as well, going even so far as to have Turk state that he finds all women JD sleeps with automatically disgusting (and this is confirmed with a brief POV shot), when this is not only contradicted in previous seasons by various moments he shares with Elliot (including a sex dream that provides the crux of an entire episode), but just a few episodes later when JD makes a big deal of Turk sleeping with one of his former college girlfriends. Of course, this inconsistency is just for a throwaway joke that's never followed through on, while JD's behavior with Kim concerning her pregnancy and his brief flirtation at infidelity with an engaged Elliot completely rewrites his character to provide fodder for a seventh season that almost didn't happen.
Between the first season and later seasons, one aspect of Elliot's character was noticeably changed as well; early on in the series, Elliot is not particularly fond of children or babies and shows little to no desire to interact with them. For example, in the first Christmas episode, Dr. Kelso makes a very sexist remark to her about how almost all female doctors end up in women's specialty, and she later tells him that she is completely uninterested in babies, doesn't coo when she's around them or melt when she sees them, and has no desire to make them her life's work. By the end of the episode, she does feel some affection toward a baby born to a teenager in the care of her hospital, but it seems clear, and it would make sense, that she still has not completely changed her views. Contrast this against the Elliot of later seasons, who is extremely jealous of any pregnant women around her and wants her boyfriend, Keith, to pretend they're not wearing a condom while they're having sex so that she can fantasize about getting pregnant.
Elliot: Oh my god Turk, I am so sorry, I just love babies so much. Keith: It's true. Sometimes she makes me wear a diaper. Elliot: Keith, private!
A far more serious derailment regarding Elliot is the fact that she goes from believably insecure to insanely neurotic and shrill. She also becomes thoroughly insensitive of other peoples' feelings for a couple of seasons (until Carla finally calls her on it after breaking up with Keith.)
Early seasons have Jordan as a bitch, but still undoubtedly a good mother who is very concerned about her son's safety and spends most of her time taking care of him and making sure he and Dr Cox have a good relationship. In later seasons, she's completely neglectful of her children and wants them to despise Cox, a complete 180 of before, just to exaggerate her vindictiveness. she and Cox also started out nose-to-nose in nastiness, but now she's kilonazis more evil than him.
This isn't as glaring an example, but what on earth happened to the deep, emotionally intelligent Booth of earlier seasons? And why do the characters - and writers - treat Fratboy Booth the same as they did his good twin?
This could be explained by his brain tumor, which did cause problems with his behavior. The fifth season premiere seems to establish that, since his coma dream, he's returned to his emotionally intelligent ways.
Not to mention the way that Bones herself has gone from a relatively normal, if slightly socially awkward, woman into someone who is essentially socially retarded.
Arnold Spivak from Murder One was portrayed in the first season as somewhat socially awkward but skilled and mature lawyer. In season two, he becomes a whiny brat and approaches full-on Butt Monkey-dom in the final episode when he completely blows a job interview.
The Doctor from Star Trek: Voyager started out as gruff but cool-headed and efficient, seeking ways to expand his horizons and learn about the world outside his programming. Toward the end of the series, however, he was frequently portrayed as an egotistical Jerk Ass so narrow-minded as to be incapable of seeing anything outside his own wants and needs. Unsure if this counts as Derailment, as there were still episodes at that point that stuck closer to his original character.
Forget the doctor. What about Janeway? She started off well, but got more unstable and increasingly messianic about her own infallibility as the series went on, with no real explanation. Kate Mulgrew has publicly stated that she believes her character was suffering from some sort of mental disorder. Chakotay, by comparison, increasingly lost his spine.
A lot of Voyager's retroactive, out-of-the-blue attempts at character development might count as this, but none of them ever lasted more than one episode.
When you remember that the series suffered from very inconsistent writing and overuse of the Reset Button then none of this is surprising. The Doctor could at least be explained by how he was a constantly evolving character similar to Data, who was trying to discover his own place in the universe. While Janeway seemed to go from "Being ready to get home no matter the cost." to "Mother hen not willing to risk her crew for any reason.". Robert Beltran went on record saying how he intentionally played Chakotay wooden due to how unhappy he was with the writing and how his character was used.
American Gothic has the most obvious candidate of Gail Emory, Chickified from an Action Girl and Determinator into a Distressed Damsel, but some might term Dr. Crower's descent into madness, Merlyn's turn as an avenging angel, and Caleb's descent into darkness as examples of Character Derailment as well rather than Character Development. At times Merlyn could also appear Flanderized and Selena seemed to suffer from Villain Decay. The only character who could (subjectively) be said to grow and develop normally is Ben Healy...while Buck always stayed true to his roots.
Heroes. Circling the drain on its third season has derailed virtually all of its characters.
Mohinder Suresh went from a curious, intellectually hopeful scientist, whose lack of conventional superpower brought complexity into his dynamics with other characters, to an aggressive superpower-junkie with hardly a personality beyond his amplified ego. This is also the result of plot derailment, when the show went from being an exploration of accelerated human evolution to a simple display of Mad Science.
Claire went through an interesting character arc in the beginning. She had a Cursed with Awesome phase, found out her dad works for a secret company. That was all good in season one, made sense. Season two onward, it seems like she never resolved any of those issues and bitches more than any other character about her power, including the ones that kill people by feeling an emotion. The details change every Volume, but Claire's story arc always basically boils down to "I can't trust my father!" regardless of whether or not it makes sense.
Volume One - finds out Noah hunts down people like her for a living but reconciles when she realizes just how much he risked to keep her and her powers safe from his bosses.
Volume Two - gets angry that Noah lied to her and her mother, continuing to work as a secret agent trying to bring down The Company. This was especially hypocritical since Noah was riding Claire hard about the importance of not doing anything to attract attention. So joining the cheerleaders at her new school is too risky but his globe-trotting and killing his old teachers is okay?
Volume Three - after going through what amounted to Mind Rape with Sylar, wants to start using her powers to fight bad powered people like Noah does. Noah wants her to live a normal life.
Volume Four - Claire takes Noah to task for working with Nathan and the government-run Company and allowing all the decent people he knows who have powers be illegally imprisoned while making deals to keep her safe.
Season Five - Claire and Noah actually seem to be reconciling... until the new Big Bad tells Claire that Noah once killed a man in front of his young daughter and Claire suddenly goes back to being convinced her dad is a monster...
Sylar, the original villain and recurring Bad Ass since season one, arguably got the worst treatment in season 3, which is really saying something. He turns into a good guy simply because Angela falsely told him she's his mother, naturally he believes her for no apparent reason. He dresses like a nerd and starts working for the company with the help of Noah (the only character to actually behave in character throughout the season). That is, until Arthur tells Sylar that he's his father and Angela was an abusive mother. Again, all lies and no evidence at all to back up his claims, and Sylar believes him again, and works for Pinehearst, Arthur's company, instead. Noah finally lets the cat out of the bag and somehow this puts Sylar back on track, he no longer believes he's a Petrelli, and finds a way to prove it. Of course, not before one last punch to his character, he kills Elle on the beach for no apparent reason. This was so close to adding to him being a Magnificent Bastard, until it was revealed it was never a trick or a trap, he truly believed all the lies.
Adam Monroe, who transforms from Magnificent Bastard to sniveling coward in a single scene, all as a pathetically transparent attempt to build up the new Big Bad, Arthur Petrelli.
And it's continued to happen even AFTER he's dead. He's gone from being the founder of the Company to just some guy who the founders met after it had already started up way back in 1961, completely ignoring all that stuff in season 2 about Adam being the one who brought them all together
Maury Parkman, previously a ruthless, scummy creepy monster who cared only about himself was retconned into Arthur Petrelli's snivelling lackey and then died...demanding that Arthur not kill his son. His son who he himself tried to kill only the previous season.
As if the individuals weren't bad enough, they did an episode that derailed all the characters at once; Villains, it played like a piece of bad fanfiction by somebody who never even heard of the show. Sylar was the good guy nerd, even though this was supposed to be after his fall when he started acting like a serial killer. Elle was gentle and kind, and hated that she had to feed somebody to Sylar. Agent Thompson released Meredith out of pity that she lost her daughter, even though he was likely the same person that took her away in the first place, and is the same Agent Thompson that nearly killed Matt simply because Matt wanted to be left alone, the same Thompson who tried to take Claire away from Noah. Nathan's line in the episode "Thank God Dad never found out what his sons were about to do to him," despite the fact he yelled precisely what he planned to do to Arthur to his face and that Nathan hates his father, all in the same damn episode. Linderman feels sorry about Angela not knowing that Arthur plans to kill Nathan, despite just coming back from trying to kill Nathan himself. Speaking of which, they turned Angela into a fucking housewife.
Most of this was positive character development. It did make a few of the characters more likeable.
On Happy Days, Fonzie was originally a minor side character, who was a genuine juvenile delinquent. He had dropped out of school, and in one episode went back to school, before dropping out again because he decided he couldn't be bothered. He was also shown to "date" multiple women, often without them knowing about each other. Once Fonzie became a major (if not the major) character, this changed. A few years later, suddenly he is able to graduate high school with Richie and the gang after all, because he went back to school (for a second time?) and took night courses or something. He is also seen to be preaching to Chachi that he should be honest with the women he dates and let them down easy, etc. This character transformation has been referred to as "Father Knows Best in a leather jacket and sideburns".
Similarly, Hawkeye of M*A*S*H went from being a mischievous goof-off obsessed with women to an unfailingly moral Marty Stu preaching at us.
Hawkeye gets derailed any time the plot requires him to act differently from his normal character. In some episodes, he talks about the sacredness of his Hippocratic oath. In other episodes, he falsifies medical documents, makes a colonel think he's sicker than he is to get the colonel to go home to the states, performs unnecessary surgery, and on several occasions, drugs patients who don't need to be drugged, including Frank. One of those times was because he wanted to throw a party and Frank was in the way.
General Hospital has Damian Spinelli, who went from the pot-smoking lecherous surfer guy who happened to hack computers, to a shy, anti-social, sad little geek who had no self-confidence and acted as a prop for Maxie Jones, through massive woobification.
How about the massive character derailment in Maxie to prop Spinelli? The pairing ruined them both. Maxie went from a bitchy, snarky, basically a Jerk Witha Heartof Gold who, while still sympathetic, actually enabled a married man's drug addiction by carrying on an affair with him, to a shrill-voiced, hopeless, stupid harpy who constantly spouts that Spin is her "essential person" and how OMG LOST she'd be without him.
As the World Turns saw many instances of character derailment towards the end of its run, not coincidentally around the time Jean Passanante became head-writer. Instances included the transformation of Adam Munson from a down-to-earth character with strong moral compass into a sociopathic rapist to prop Gwen Norback; turning Dusty Donovan into a Stalker with a Crush against Emily Stewart (someone who'd kidnapped him and tried to kill him); turning Paul Ryan into a Holier Than ThouMorality Pet against other characters (after he'd stolen his little sister's baby because the baby's father was his mortal enemy); and turning Carly Tenney into a manipulative, neglectful mother to her three children.
A weird case where character development was actually happening but the effect didn't work - Season 15 of Doctor Who was edited and aired out of order to allow K-9 to be permanently added to the cast despite many of the episodes already having been shot. This had some seriously negative effects on Leela's character, as what had been a gentle character arc about her slowly learning and growing became a bit of an inconsistent mess of Aesop Amnesia. A very minor example is that the Doctor's influence was going to have caused her taste in fashion to change, from very skimpy Nubile Savage wear, to a mashup of it with the Doctor's Regency-influenced style (a fitted leather dress and her hair being worn curled) - this fairly significant development is reversed at the end of the episode with a short scene of her going back to her old look.
Somewhere between "Castrovalva" and "Four to Doomsday", Adric became a gullible chauvinist (the latter is particularly odd seeing as he's spent so much time with Romana without ever commenting).
In Warriors of the Deep, Dr. Solow gets confronted with the Myrka and makes the fatal and unfathomable mistake of deciding to karate-kick the beast.
House has Cuddy, who went from a strong, sane woman (with a Guilt Complex) who could run her hospital well and still deal with House relatively well to an incompetent, baby-feverish, overly-emotional boss who acts like a child. She still has her moments but Cuddy, sweetie, when House is acting more mature than you, then I think that we've got a problem.
Not to mention the way she's morphed into a woman whose sole obsession in life is being a mother, and she seems to resent her career for pulling her away from her adopted baby. Although she's getting older, her level of obsession is still a bit ridiculous and hard to swallow.
Dr. Foreman was perhaps the earliest victim of Character Derailment. In the first season, he was ambitious and unsentimental, but had several Jerk with a Heart of Gold moments to make up for it. One or two episodes into the second season, he stood back and did nothing while a patient's heart failed because he didn't like the guy. Later in that same season he pulled a professional dick move on Cameron, then when he was mortally ill, stabbed her with a dirty needle in a desperate attempt to get her to save him. To say he Took a Level in Jerkass would be an understatement.
The second series of Robin Hood had character development from everyone, but most notably Guy of Gisborne, who, by the penultimate couple of episodes, is risking his own life to stand up for what he believes in and to defend others; something he would never have done back in the day. This is mostly because of his love for Marian, and he repeatedly defies the Sheriff for Marian's sake. Marian shows Guy friendship, despite working against him in secret, and is very very good at keeping her double-agent status secret. In the finale, Marian suddenly begins flailing about like a muppet, yelling "I LOVE ROBIN HOOD! I'M GOING TO MARRY ROBIN HOOD!" Guy then stabs her through the gut and rides off. Two characters derailed for the price of one!
It seems to run in the family. The third season introduces Guy's sister Isabella, who seems intelligent, helpful and rational. Robin begins a relationship with her, but when she encourages him to run away with her, he tells her that his loyalties lie with England and the mission. She responds by picking up the nearest sword and trying to kill him. As you do.
On Dirty Sexy Money, Lisa George is shown during the first season and first part of the second season to be a kind, reasonable woman — flawed, yes, but, you know. A human being. In the show's second season, the writers seemed to realize they could hardly justify Nick just leaving her and hooking up with first love Karen Darling. Thus Lisa became an unreasonable shrew who harped on Nick at every turn. Nick also suffered slightly.
The Playboy TV series Boy Nexxt Door has the main character start off as a somewhat nerdy, goofy, naive guy who still had plenty of successes and quite possibly a bright future ahead of him in the adult film industry, and after a few seasons quickly derail into the biggest loser imaginable, who is going to spectacularly fail at whatever he tries no matter what it is, and who women find laughable and pathetic pretty much automatically... nevermind that the whole catalyst for the series was the fact that he had the ability to not only attract numerous hot girls, but actually convince them to let him film it. Apparently with Playboy magazine's decay into a Maxim clone, the network had to follow suit and make anyone even slightly uncool objects of total derision.
Nearly all the characters on The West Wing got derailed after John Wells took over from Aaron Sorkin. Examples include:
Bartlet started as a kindhearted, twinkling-eyed, fatherly hero with a bit of a temper and ego problem, who valued people who disagreed with him, loved his friends and staff, and was quite the rebel when it came to political etiquette and tradition. He was even given nice character development, becoming darker, sadder, wearier, and given a tiny touch of Knight Templar, which he dealt with by being even funnier and more off-the-wall than before. After season 4, he became a weak, doddering, cold-hearted, indifferent, rigidly by-the-book president political hack who was obsessed with legacy, isolated himself from his staff, and fired friends for disagreeing with him. And even that was inconsistent, with the writers constantly attempting to give this new, bitter, flaccid character moments of awesome that never rang true.
Leo going from the President's wise, gruff-but-kind, compassionate right-hand man to useless, trigger-happy old goon who acts like a complete Jerk Ass towards the staff, bullies and rejects Josh (who thinks of Leo like a father), spouts jingoistic, morally indifferent bullcrap at all possible opportunities and once advocated planting false evidence to start a war. This from the guy who used to be the moral center of the White House...
CJ going from sassy, suave, pragmatic, razor-tongued goofball with a streak of The Lad-ette and total cluelessness with romantic relationships to drab, bitter, morally-superior, constantly nagging woman-in-charge who hated politics. There's a vague attempt to present this as a "logical" result of her promotion to Chief of Staff, which makes no sense, because Leo was certainly never a bitter man who hated politcs. Prime example of a Double Standard ï¿½ a woman can't handle authority unless she's a Jerk Ass, and god forbid she enjoy politics, which is a man's arena. While CJ's promotion was off-key, her derailment began with the rest of the characters - at the beginning of season 5, long before her new job.
Josh losing his humor and sweetness to be shoehorned into the de facto lead character of the show, who happened to be deeply screwed up and cruel instead of Troubled, but Cute.
Sam, who left a miserable plush existence and a six-figure salary at a prestigious corporate law firm in order to campaign for Bartlet, vanishing and going back to corporate law after losing a House election, effectively canceling out everything that had defined his character up to that point.
This particuler example actually happened during Sorkin's tenure, and was done because the actor playing Sam bolted mid season, and the character had to be Put on a Bus. Even so, Sorkin could have found a better way to write him off.
This is particularly glaring considering that the fourth season had been largely building to a perfectly natural way to both get Sam out of the show and keep in line with his character's political interests and leanings; namely, the congressional seat he'd been running for all that season.
Toby turning traitor on the White House and leaking the existence of a top-secret military shuttle. And this was after his relationship between with Bartlet, which had always been one of Undying Loyalty and Sarcastic Devotee, became practically toxic (although given Bartlet's own derailment, it's hard to blame Toby too much). Richard Schiff, the actor who played Toby, actually called this out as an example of Character Derailment after the show's run was over.
Even Donna got this, though less so than most other characters. After steadily developing maturity, charisma, self-confidence, contentment, and political savvy for four seasons straight (while the UST between her and Josh ramped up to the breaking point), Donna quickly began to lose nearly every quirk she possessed one by one and for some reason became very unhappy.
Hoynes got derailed as, one can only assume, part of Aaron Sorkin's bid to write Wells into a corner. The VP was certainly more of a political machinist than the rest of the cast, but he was still a smart, well-meaning guy. Certainly not one to tell his mistress state secrets to impress her!
While it's YMMV as to if the events of Life on Mars where character derailment or not for Hoynes, it was the events Full Disclosure really knocked Hoynes off the rails.
Will Bailey. As Sorkin wrote him, Will is a deeply committed, passionate idealist who's in it because he genuinely wants to make a difference. During the election in season 4 he continues campaigning in earnest even after the candidate he represents has died. For everybody else, the campaign is over. There is absolutely no way that the Democrats can win this particular district, and the fact that Will's still out there trying to win votes has made him the laughing stock of the entire party, but he pushes on despite it all and in the end he very nearly succeeds. He's then offered a job at the White House as a speechwriter for the President. It's his dream job. This is where he wants to be, and because of that he'll endure hazing, bullying, and the entire speechwriting staff quitting and leaving him with an impossible amount of work. Will is not a quitter. Not, at least, until John Wells takes over the show and Will receives a plush job offer from the Vice President - which he accepts, leaving his self-described dream job because he's decided that Toby is a bit of a difficult boss to work under.
As Dexter progresses, moving away from the novels and changing many of the character details in order to be a more palatable show, the main character has taken on a family, gotten married and had a kid of his own too. Not to mention his regular "fantasy moments" where he has open conversations with his foster father Harry, as if that character now serves as the link to Dexter of the first few seasons. Although he's still dark, and still kills occasionally, he seems a lot less driven than he was in past seasons. It's almost as if he is slowly being demoted from a serial killer to a troubled sociopath to bring in a wider audience, losing the substance that makes it such a unique premise. Not just character derailment but show derailment, especially if you count the fact the show now has a MUCH bigger focus on romantic relationships than it did in the first season and the increased reliance on stock plots/characters and the odd Cliché Storm.
Some Dexter fans are more accepting to the tonal shift than others. Part of this may have to do with avoiding the It's the Same, Now It Sucks syndrome that usually applies to series during its third year. Another reason may be the TV writers dodging the supernatural bullshit that plagued the third book, Dexter in the Dark (though Dexter got married in that novel too). Still, although Dexter has come a long way from being the isolated loner he once was, his urge for killing has not waned THAT much, even though he's now a family man. And given the headaches Dexter had to endure during season two after scuba divers found his submerged corpses, and given the increased focus to his family, he needs to be less impulsive. While season four (the season criticized above) suggested that Dexter would reach a peaceful reconciliation with his murderous desires, after Rita's brutal death during the equally polarizing ending, he might go towards a much darker road (and subsequently, the show will follow).
Not to mention Dexter used to go only after criminals that were able to get around the law. Now he goes after criminals the police are currently after, seeing as if it is a game of challenge to beat them.
The Lieutenant of Miami Metro PD at the end of season 1 and beginning of season 2, Esme Pascal, got hit by this between seasons. She's shown to be a Reasonable Authority Figure, a very competent cop, and a pretty good person. While she did have Maria La Guerta actively trying to make her look bad, it doesn't change the fact the woman became a complete idiot, falling for very plain ruses and jumping to conclusions very publicly.
Desperate Housewives featured a particularly egregious example for Carlos. Up to this point the worst thing Carlos has done is have an off-shore bank account; he has otherwise been one of the more sane, generally balanced, characters in the show. Season six has seemingly found this depiction dull, and decided the role of Carlos will now be played by a sexist pig and world-class Jerk Ass. He has spent the season making it clear he will not tolerate a pregnant woman on his staff; and in the last episode has harassed, insulted, and fired (after giving her a choice between work or seeing her daughter's play) Lynette.
Sam. Looked like growing up a little in Season 3. Turned into a sociopath with no redeeming qualities in Season 4.
Then falls 'in love' with main character Freddie (who she has spent 4 seasons beating on) in the final episode.
In Season 3 of Gossip Girl, the character Damien was slowly being developed to be more than just a cardboard cut-out Bad Boy. He seemed to have genuinely developed feelings for Jenny and was understanding and caring. However, when Jenny decided not to have sex with him because he thought being a virgin wasn't a big deal, he immediately left and made a comment about her being a "kid". This felt very abrupt and conflicted with all his previous character development.
However no Gossip Girl character has been hit harder by this than Trip van der Bilt. When he first appeared he was a sweet, decent guy, one of the few people in Nate's family who actually seemed to be a genuingly good person. Then within the course of three episodes he cheated on his wife with Serena (while claiming he was in the right since his wife Maureen pulled a dirty move to help him win an election) and then got into a car accident with Serena in the car and moved her unconscious body to the driver's seat so that no blame would fall on Trip himself. His behavior appears to have been written in simply so that Nate would look swell in comparison.
T-Bag. Oh T-Bag. Throughout Prison Break's first three seasons, he was a Magnificent Bastard who knew how to manipulate every part of his environment, whether in Fox River prison, a well known fugitive, or in Sona. Unfortunately, on the first episode of season 4, T-Bag made many really dumb mistakes. Like, for one, pursuing revenge against Michael Scofield for the hell he put him through. It's kind of understandable, considering that T-Bag didn't have it easy, but extremely stupid when you realize Michael's also the reason he's out of prison AND has millions of dollars, courtesy of Westmoreland's stache. To top it off, he had like three hot-ass Latino chicks in his bed. And, to make matters worse, soon after T-Bag hires a guy to transport him to the US, he and his buddies seize the opportunity to steal all of his money and throw him into the desert. And as a result, T-Bag had to resort to cannibilism with his Hurley-sized partner to survive. Sigh, To be fair, when the writers had to gather every major character again to start the Scylla hunt by arresting them, they did it in a pretty unconvincing way, seeing as they all successfully dodged police for most of season 2. But with T-Bag, it was so blatantly out of character, it ruined him the entire season. His de-evolution only gets worse from here.
In Ace Lightning, secondary character Heather Hoffs was, in the first series, a somewhat highly strung, but determined, hard working, confident girl. In the final series she became a nasty, self centred bitch. (This was possibly done to show off how great the series new Canon Sue was.)
Ted from How I Met Your Mother. Sometime between the fourth and fifth seasons, he went from being a hopeless romantic to somewhat of a shallow womanizer, never staying with any girl for more than one episode. This also contributes to the show's Arc Fatigue, with this phase essentially putting the story on hold for (at least) a year.
He was left at the alter, and during As Fast As She Can he explains that his romantic side died that day. In fact, he often notes how, during that time, he was a bit of a jerk. He tends to get back on his romantic track later.
Stella's boyfriend/fiancee, Tony, had a serious, off-screen amount of derailment in "The Wedding Bride." Before the episode, he and Stella had been shown to have a relationship that went sour due to having a child and being too young. Right before getting married to Ted, though, Stella runs off with Tony. Later, though, Tony sees what this does to Ted (in the aforemetioned "As Fast As She Can") and dumps Stella. Ted, however, gets Tony and Stella back together, and Tony even gets Ted his job as a college professor. Essentially, Tony is a bit of an idiot, but is wholly good-natured and is very friendly to Ted and believes that he wouldn't be happy now if it wasn't for Ted getting him back with Stella. So why would he then make a movie based on the incident where Ted is portrayed as an evil, nasty, creepy jerk-face?
Ted is an Unreliable Narrator and, since absolutely everyone else - including Ted's friends - loves "The Wedding Bride" (which seems at odds with the Stylistic Suck version we see), it's possible that he's mis-remembering the content of the movie and is simply incensed that Tony wrote the movie at all. The whole point of the episode is Ted admitting he has a chip on his shoulder about being left at the altar.
It's also probable that as a first time screenwriter, Tony encountered of few tropes on the way to getting the film made, including some Executive Meddling.
Also, Ted's high school friend Punchy. In season 6, he was a Nice Guy who worried about Ted leading a sad life after being left at the altar and asked him to be his best man at his wedding. Then when the wedding happens in season 7, we found out Punchy "actually" hired Ted as his best man is because the latter always cries in the middle of a toast. He makes fun of him and even does autotuned videos. It really feels like a Retcon.
Barney's gay brother James goes from a Nice Guy who gave up the promiscuous lifestyle to marry someone he loved and have a family to a Jerkass who cheated on his husband repeatedly and worked out multiple times a day despite being married with kids that needed him, "just in case he's ever single again". Possibly justified in that we never saw much of his life on-screen, but what we did see suggested that he was happy, stable, and committed and never really foreshadowed otherwise.
Most of the main cast has gone through their own form of Derailment, but the most prominent of them is Robin. She began the series as a young independent woman who had tomboyish tendencies that set her apart from Lily and other female characters, but was still not afraid to show her feminine side. We even learn later on that she was raised as a boy by her strict father, so when she moved in with her mother she started embracing her feminine side because that's what she wanted. All of these aspects of her personality are suddenly forgotten or exaggerated in Season 9, where Robin is suddenly so much like a boy that she doesn't understand the concept of crying, or having empathy in general. One notable episode in the season focused around the problem that Robin's boyish personality was the reason why she doesn't have any female friends, even though in every season before she was shown interacting with other females outside the group with no issues.
Barney got hit with this in the series finale. After several seasons of Character Development and maturing from a serial womanizer to a (mostly) mature man marrying the woman he loves, it's revealed that he and Robin got divorced three years after marrying because she was too devoted to her career. After this happens, he goes back to his old self, creating a new Playbook and trying to have sex with a different woman each day for a month. Though he apparently grew out of it when his daughter was born (as a result of one of his one night stands), this Snap Back was one of the many reasons most fans despised the show's ending.
Hannah Montana. Miley started out as a kind, normal, unpopular teenage girl, and now she always has to get what she wants and assumes that everybody loves her. In the first season, Lilly was a tomboy who never wore skits and always skateboarded with her guy friends, but now, there isn't an ounce of tomboy left in her and she always cries. Oliver's become weak and whiny, not to mention a pop sensation touring the country. Jackson can't score a date for his life now, when in the beginning, he wasn't nearly that pathetic. Robbie is now a hair obsessed freak who acts like a child. Miley, Lilly, and Oliver are all popular and famous now, which totally defeats the purpose of the show in the first place.
Ben from The Secret Life of the American Teenager. He went from a caring, sensitive boy that helped/supported his bitchy jerk-sue girlfriend throughout her pregnancy(A baby he was NOT the father of, to boot. She got knocked up before she met Ben) to an obsessive, sex-crazed jerk whom when he got another girl pregnant all he cared about was Amy and seemed to partially not care that she wanted to get an abortion. Even after she dumps all he seems to care about is Amy. Though, he has gotten better and seems to care more about being a dad now. Some fans believe this was so "Ramy" would seem responsiable. You know, switch their personalities and make been the Marty Stu.
Morgana from Merlin. Her turning evil was always a Foregone Conclusion, but the way the writers decided to go about it has been baffling. After a year in the company of her half-sister Morgause, Morgana returns to Camelot and tries to assassinate King Uther. Her resentment toward him at least has been built up properly, but why the writers felt the need to have her spitefully goad Gwen over her relationship with Arthur (the former being her dearest friend who has never done anything to her and the latter being someone she loves as a brother), plot to deprive Arthur of his throne (who in the past she's recognised as "a better man than your father" and recently discovered is her half-brother) and stab a knight of Camelot when on the brink of being discovered, throw his body from the parapets, cold-bloodedly finish him off with poison on discovering he survived the fall, and all with a perpetual Evil Smirk on her face is completely at odds with the Morgana of the first two seasons.
To make things worse, when she was finally crowned Queen of Camelot, her punishment for the citizens not pledging allegience to her was to burn their crops, and she took pleasure at the idea of their children starving. From this point on, there was not a hint of the old Morgana again. She had completely become the man she was fighting against, but perhaps that was the point?
Uther himself when his spirit returns in The Death Song of Uther Pendragon. He coldly dismisses Arthur as a weak and foolish king before attempting to kill him "for the good of Camelot". While it's true Uther often criticized Arthur as being a naive Wide-Eyed Idealist, his unconditional love for Arthur and Morgana -often to the point of blinding him to reason- was by far his biggest redeeming quality in life. The idea that he would knowingly try to kill his own son was ludicrous. Some try to Fan Wank this as Uther having Come Back Wrong, but this is never indicated in the episode itself.
Some time between seasons three and four, Gwaine went from a wise rogue to a complete buffoon. His close friendship with Merlin has also seemed to have gone missing.
Bizarro, a rare evil to (relatively) good example. In his first appearance, he takes lives with psychopathic delight, but in his second appearance he saves Chloe and Jimmy from a bomb and his Last Words to Lana before she killed him with blue kryptonite is "I love you", which makes him a little too sympathetic. Although it might be an effort to reverse the Adaptational Villainy he had in his first appearance, as Bizarro is traditionally confused but not irredeemably evil.
Many thought that Davis Bloome's sudden last minute Face-Heel Turn into being unrepentant murderer at the end of the 8th season of Smallville was this. Starting out as the Doctor Jekyll of a Dr. Jekyll And Mr. Hyde character, Davis was progressively forced to do more and more extreme and immoral things to keep his Mr. Hyde monster side (Doomsday) under control. He had been abandoned on the streets has a child and had grown up in foster care and become an EMT where he met and fell in love with Chloe Sullivan. The attraction between them was mutual despite her engaged to be married at the time. As his condition worsened he finds out to his horror that his purpose in life is destroy the world. Davis is shown again and again as a good man by himself but constantly plagued with the destruction his alternate personality causes and being forced to cover it up turning more into an Anti-Villain or Anti-Hero type. So what happens when they finally manage to separate the human Davis from the alien monster of Doomsday? Oh guess what? Despite his anguish over Doomsdays' murders and even two past attempts to kill himself to prevent it, it turns out Davis is just as much a psychopathic murderer as Doomsday and he tries to kill Chloe of all people. To say that this sudden Face-Heel Turn (literally in the last five minutes of the last episode of the season!) didn't make any sense in the context of Davis' entire characterization up to that point would be a gross understatement.
Chloe Sullivan arguably has some of this around season six, from being the determined and independent girl whose only weakness is Clark Kent. Who could blame her for being soft towards the Man of Steel? But then Jimmy pops up and she rolls right over for him. Like the starting scene of Quest - it just comes out too mushy for her character.
Later, she went from being a Nice Girl and Unlucky Childhood Friend to Clark to a morally ambiguous Manipulative Bitch who cheerfully did "the right thing" and blamed Clark for all her mistakes. When did it happen? Season 8, Episode 20. By the start of the next season she's gotten her husband killed through a series of magnificently stupid decisions, put all the blame for it on Clark, and decided that murder and endangering her friends are okay if they get the job done, something she never would have done before. Essentially the show took one of its most likeable characters, and turned her into Lana Lang 2.0. Maybe they thought people actually missed Lana?
Dead Like Me's TV movie Life After Death features copious amounts of this, with Daisy and Mason dealt a particularly bad hand.
In the DVD movie a lot of Daisy's character and her growing relationship with Mason was dumbed down. She's now a flat fluttery headed drunken slut instead of just pretending to be, and she and Mason seem to be back to square one, even though he gave her an engagement ring in season two, and in a very small way she accepted it. I guess we can only assume that something happened in the years between the show and the movie that changed them both.
Mason, the same guy who started drinking heavily upon realizing he royally fucked up the birthday party of a little girl whose father was his reap and gave a small boy about a quarter of his Halloween candy upon realizing the boy would die (and after having bragged about how big his load would be) now doesn't give two shits that a dead man's soul is trapped in his body.
George also gets this retroactively, instead of merely ignoring Reggie as a little girl she was downright abusive of her.
CSI: Crime Scene Investigation has Jim Brass. A lot was made in the early seasons of how Brass was upstanding, clean and refused to become a Dirty Cop. But the end of season 11 had him covering for Ray Langston when Ray killed the already handcuffed Nate Haskell. Brass pocketed the flex cuffs so it wouldn't be obvious right away that Ray killed the guy when he couldn't defend himself and was no longer a threat. It's true that Brass probably saw it as helping a friend who was pushed over the edge by a deranged killer, but many still see it as a derailment of Brass's character.
Some of this also got thrown at Grissom, as some fans had a hard time believing he'd risk his career for his relatonship with Sara. Then, season 13 came along and fans started saying Grissom got derailed for what was said to have happened in "Forget Me Not" despite not being there anymore.
CSI NY has been accused of this with Danny, with fans saying pairing him up with Lindsay changed his character far too much from the earlier seasons.
Joey from Friends started the show out as a Deadpan Snarker who was around the same level of intelligence as everyone else in the gang. In the middle of the series he became a little Book Dumb but still fairly competent - note one episode where the knob on the radiator breaks and causes the apartment to overheat, and it's Joey who thinks to turn it off from underneath. By the end of the series he is completely stupid, not being able to learn French properly, can't understand simple maths, believes any outrageous story one of the gang makes up and even becomes a Man Child who refuses to share food and has to sleep with a stuffed penguin every night.
Likewise Monica was the most sensible and rational member of the group at the start of the series. She eventually became highly neurotic, scarily obsessed with cleanliness, overly shrill and competitive (to the point where she was in competition with herself).
Ross and Phoebe went through this, too. At the beginning of the series, Ross was portrayed as the most grounded of the three guys. Sensitive, kind and respectful towards women. Later in the series, he became a loud, obnoxious Jerk Ass who was every bit as lecherous as Joey and Chandler. Meanwhile, Phoebe started out as a ditzy but sweet hippie chick and easily the nicest of the three female leads. After her pregnancy arc in Season 4, she became a lot meaner and nastier, often saying and doing things she would have never done during the first few seasons. At the same time, her wackiness was greatly exaggerated, crossing the line from adorably quirky to downright messed up.
Many would argue that Ross was a possessive and lecherous jerk even during the first few seasons, only a bit more subtle about it at the time. There are many instances during the first few seasons where Ross is demonstrated to be quite selfish and possessive (witness his numerous attempts at depriving Rachel of men she was interested in before their Season 2 hook-up).
Game of Thrones: This crops up occasionally. It's usually because of adaptational differences from book to screen.
Doreah gets a very abrupt Face-Heel Turn. After a season of being a scrupulously loyal servant and supporter of Daenerys, she suddenly betrays her and gleefully murders one of her own friends. Because her character simply dies randomly in the original books, this was apparently seen as a more interesting end to her character.
Shae takes a sudden veer into Clingy Jealous Girl on Tyrion without much preamble, feeling threatened for no real logical reason by Tyrion's marriage to Sansa. This was all ironically necessary because of her Adaptational Heroism: While in the books she just a shallow opportunist who was playing Tyrion the whole time, in the show she honestly loves Tyrion, so the show has to make a sharp swerve to get her to betray him.
Phoebe from Charmed was a little scatty but good natured for most of the series, most importantly she was the most forgiving of all the sisters. She ended her relationship with Cole at the end of season 4 and tried to let him down gently. Fast forward to season 5 where all of a sudden she blames Cole for everything that has gone wrong in her life and is willing to try and kill him simply from a bad dream she had. On top of that she suddenly becomes obsessed with finding a husband and getting pregnant to the degree where she uses her premonitions on the first date to see if the relationship is going anywhere. In season 8 her characterisation finally gets fixed and she's a lot closer to how she was in the beginning with a good bit of Character Development thrown in.
Paige started out as a together and sensible enough young woman who had a social workers job. In seasons 5 and 6 she suddenly becomes a huge ditz and is seen with a different guy every episode (her actress even complained about this in various interviews). Plus she spends season 6 going through a whole stream of different temp jobs completely forgetting that she has a law degree. Like Phoebe she gets fixed in later seasons.
Meg in Supernatural. A major antagonist in season 1, and a minor antagonist in seasons 2 and 5, the writers decided to turn her into a "good" guy in season 7 by means of Enemy Mine, and even teased a relationship between her and Castiel. What the writers seemed to forget was the absolute mutual hatred that already existed between Meg and the Winchesters -Meg had, in the past, kidnapped and tortured their father, possessed Sam, and was indirectly responsible for the deaths of Jo and Ellen Harvelle, two of the Winchesters closest allies. The Winchesters, meanwhile, had killed Meg's "brother" Tom and "father" Azazel, and locked Lucifer -who Meg practically worshipped- back in Hell. This was all discarded so that they could team up against Crowley... Who, in season 7, wasn't even openly antagonistic to the Winchesters and arguably helped them more against the Leviathans than Meg herself did. The result was that Meg went from being a relatively popular character to The Scrappy, and the Winchesters looked like raging hypocrites for condemning Castiel's similar alliance with Crowley in season 6.
Sam got one of these in early Season 8 when it was revealed that he didn't even try to look for/rescue Dean, who had just spent a year in Purgatory. The same man who researched non-stop for days to save Dean's life in Season 1, insisted he was going to find a way to save Dean in the first episode of Season 2, was flat-out obsessed with getting Dean out of his deal in Season 3, turned into an emotionless terminator in "Mystery Spot," tried absolutely everything he could think of to get Dean out of Hell prior to Season 4, ultimately allied with a demon and developed a demon blood addiction to avenge him that season, and rescued Dean after he'd vanished into thin air earlier in Season 7. There was some lip service to a Heroic BSOD, but it was only briefly referenced and never shown in the various flashbacks that season. Between the "didn't try to look" in Season 8, a major lack of explanation beyond "thinking Dean was dead/we agreed to go on with our lives" (because they'd always done that before?), his reluctance to hunt again and his Romantic Plot Tumor subplot flashbacks, many fans, even two seasons later, are absolutely flummoxed as to what the writers were trying to do other than manufacture conflict between the brothers or make Sam out to be an uncaring asshole. Word of God assertions that Sam made the "mature" decision in going on with his life didn't help matters.
Mrs. Garrett in The Facts of Life: In the episode, "The Four Musketeers," the normally fair and level-headed authority figure uses her power and influence to help the girls of Eastland out, but in this episode, when the girls have earned their freedom and no longer have to be room-mates, she's saddened by the loss of friendship, so she tricks the girls into getting themselves in trouble again to reinforce the status quo.
24 gets a Broken Base over this regarding Tony Almeida's treatment in the seventh season where he was revealed to be working to kill the mastermind of the events of the fifth season's conspiracy which resulted in the deaths of his wife and unborn son. Its defenders state that after what happened to him Tony understandably crossed the Despair Event Horizon and had lost all meaning in life. Its detractors point out that in season 5 Tony previously couldn't even murder the man who directly ordered their murder because of his conscience, yet now was suddenly cool with masterminding the deaths of several innocent people in order to kill said mastermind. Not to mention that earlier in the season he showed genuine sorrow at having to kill someone he worked with, yet in the later episodes suddenly now has a cold "everyone is expendable" attitude.
Season 8 also suffered many examples of this, possibly as a result of the writers only getting short notice that it would be the final season.
Renee, who's a law-abiding FBI agent in Season 7 there to offset Jack's "do what it takes" mantra, becomes a borderline Death Seeker who is now a Distaff Counterpart to Jack. While she did show signs of breaking away from the law-abiding aspect of her character at the end of the seventh season by torturing Wilson, this still doesn't explain how she's degraded to her current state by the next season.
Taylor, perhaps the first President who's not willing to break the law, even when it might be more expedient and had her own daughter arrested when the latter was complicit in the murder of one of the seventh season's conspirators, becomes completely ruthless for personal gain, including what practically amounted to taking orders from a disgraced former President whom she had previously hated. The final line comes when she threatens to bomb another country if their President doesn't cooperate with her. While she does finally relent by the end of the day, by then it's too little, too late.
Logan, who was very remorseful in Season 6 after being the previous season's Big Bad, goes right back to being the same smug, arrogant, and evil prick he was before, if not more so. He's once again perfectly willing to murder anyone who isn't complicit with him, and this is after his previous Character Development, basically making his entire appearance two seasons prior to be a complete waste.
But unquestionably, the biggest case of derailment goes to Jack himself. While he had always been violent and willing to do whatever it took to get the job done, he becomes completely like Tony in the second half of the season, willing to murder foreign agents who don't know anything about the conspiracy and are merely doing their jobs. In essence, he succumbs to the same He Who Fights Monsters derailment that Tony was subject to, and only doesn't completely cross the line because he's talked down from completing his revenge, which would have resulted in a world war had he succeeded. And mind you, this isn't for any ideal this time around. Before, while his previous behaviour could be seen as acceptable given that he was always out for justice against terrorists before they could succeed in their own schemes, this time he's motivated purely by revenge. The fact that he was slowly becoming like Tony is not mentioned by anyone, likely because the events of previous seasons are always neglected save for minor throwaway pieces of dialogue.
In XenaCharacter Derailment gets taken to the Nth degree when the Twilight Of The Gods arc comes into play, with all the Olympian Gods being so ridiculously out of character (all of them except Aphrodite, who absolutely no one would be able to swallow turning evil) that it would be funny... if it wasn't actually just a ploy for the writers to completely write-off an entire spectrum of characters just for the sake of pushing forward the Hijacked by Jesus plot points:
when Ares first appeared, it was shown how the God of War fights. And that standard has basically stayed the same - with the exception that when he spars one-on-one with Xena in front of the Furies he doesn't use any godly powers - however, when the 'Twilight of the Gods' arc arrived, all Gods were reduced to idiotic morons with the God and Goddess of War (and wisdom) being taken down easier than the Warlord Of The Week. Ares gets the Worf Effect and all the Gods' powers, abilities and intelligence is taken down to the proficiency of mortals with pyrotechnics who have no idea how to use their powers or how to fight so that Xena can kill them all. Every single God gets villified, with their characters so skewed that everyone except Athena, Aphrodite, and Ares becomes a stock moronic evil henchmen no matter who they are, so that Xena can retain the moral high-ground (y'know, so the audience can try to forget that she is committing what amounts to genocide and that if she was in the position where someone was trying to kill her and everyone she loved, she would have been much much worse). Lucky that in previous episodes the show hadn't demonstrated that the Gods embody different aspects of humanity and the loss of godhood results in humanity going insane... oh, wait.
The very idea that the God of Love, the being who is the source of all Light, and who has love and compassion for all beings - who is the source of the Way of Love - wants another creature - i.e. Xena - to kill someone, and in fact murder all the Olympian Gods is such a severe case of Character Derailment that it borders on insulting. How stupid do you have to be to write in the plot point: "the source of all Love and compassion orders the wholesale murder of the Olympian Gods"? Bad writing at its finest.
Caroline has been one of the strongest characters on the show. However, in season five, she spent most of it complaining about, well, everything. If that wasn't enough, she had a one-night stand with Klaus, someone she hates, mere hours after celebrating Damon and Elena's breakup (Klaus is 20 times worse than Damon), and begged Tyler, Klaus's biggest victim, to forgive her only twice before yelling at him for being mad at her. Meanwhile, she wished that Stefan and Elena would reunite, but then got jealous when she mistakenly assumed that they were doing so. Before that, she stated to Enzo that she doesn't go around killing people, yet she got over her killing Twelve witches in the previous season. Also, she brutally murdered another witch to force his twin sister to do what she wanted.
Tyler was even worse. He dumped Caroline for virtually no reason after ditching her for months, went to New Orleans to murder a pregnant woman (in order to get to Klaus), tried to get a group of vampires to kill a child, and then came back home like nothing happened.
Damon is another example. In season five, it looked as though he was actually redeeming himself. Then, it was suddenly revealed that he spent decades ritually killing off an entire family without anyone noticing. Then, when Katherine ruined his reunion with Elena, he flipped out and killed an innocent person and attacked two of his friends.
But the worst one is Katherine. First off, she got so desperate that she sacrificed Jeremy in order to get the cure, raising Silas in the process. Then, she blamed Elena for all of her problems and what she lost, using that as an excuse to kill her. After being cured, she looked to be redeeming herself, only to possess Elena, and do such a poor job being her that nobody noticing at all is a testament to their lack of intelligence. She does a bunch of catty/dangerous things just to hurt people for no reason, culminating in her attempts to seduce (re: rape) Stefan while in Elena's body and leading to her getting caught. Finally, in a fit of rage and seeing Elena as having the life she thinks she should have had, she gives Elena a deadly poison so that she won't have Stefan and attack her friends in the process. Remember when Katherine was a fun villain?