In the shoujo manga Akuma De Sourou, Nice Guy Kamijou Yuucihi's character becomes derailed when he suddenly becomes a bit forceful towards Kayano. This doesn't really match his original character, and was obviously done so that Kayano's decision to go with Takeru is justified. The saddest part is that, even when he turns more aggressive, he's still a much nicer guy than Takeru.
In the shoujo manga Stepping On Roses (Hadashi de Bara wo Fume), Nozomu was originally a sweet gentleman who had been the merciful one that donated money to Sumi when her kid sister got really sick. However, in order to justify that Sumi should stay with Souichirou, his character becomes derailed as he suddenly becomes possessive and eventually an out-and-out Yandere for her. As if to make everyone no longer have any doubt who Sumi should end up with, he eventually goes so far as to try to rape her, only to decide to commit a double suicide with her when the place gets set on fire.
Happens in Kimikiss: Pure Rouge. At first, Kouichi's childhood friend Mao was happy to see him off with Yumi, and Mao also gets a boyfriend of her own. Suddenly, a couple of episodes later, Mao decides that she really does like Kouichi after all, and drops several big hints on Kouichi, even in the presence of Yumi. Even after promising to see each other after Yumi moves away from their hometown, Kouichi suddenly loses all interest in her, and decides to break up with her to be with Mao. His regret over the incident only lasts five seconds.
Kunihiko Ikuhara has been accused of having done this in the Sailor Moon anime. Since he's an open Yuri Fan and has said that he doesn't like the character of Usagi's boyfriend Mamoru, many fans have accused him of deliberately reducing Mamoru's powers and importance to the plot so Usagi would look better with other female characters.
He cheerfully admitted to repeatedly killing off Mamoru for this purpose, but of course the producers would always mandate that Mamoru be brought back, much to Ikuhara's irritation. Then again this isIkuhara we're talking about, so who knows if he is really serious.
Comicbooks
The Polaris/Havok/Nurse Annie triangle during Chuck Austen's run on Uncanny X-Men. While Lorna has never been the most stable of people, Austen blew her mental issues into full-blown bitchy psychosis to justify getting Havok together with the nurse he telepathically dated while in a coma (caused by her telepath son, Carter, who wanted a daddy and wasn't above committing Mind Rape to force Havok to be his daddy). This ended with Havok dumping Polaris at the altar for Annie, and Polaris flipping her shit and attempting to kill them both before Juggernaut knocked her out. Polaris was put into psychic therapy with Professor Xavier; Havok and Annie ran off to Paris together, seemingly supported by all the other X-Men. Austen wasn't at all shy about telling people that Annie was a stand-in directly modeled off his own wife and that he envisioned himself as Havok; this did nothing to help fans accept the pairing... especially since, after all this, Annie was throwing herself at Iceman while still with Havok! It's no wonder that Austen, upon leaving the book, put Annie on a bus out of town with Carter to protect his much reviled character from future reprisals. Though Austen did have her last scene feature a nebulous scene where it's implied that Carter has hooked up with Elias Bogan, a disembodied telepath who liked to trick kids into selling their parents to him to use as slaves to rape and torture for his own sick amusement...
Chuck Austen loooooves this trope. Exhibit B is his run on The Avengers, in which he pulled his trademark three-way Character Derailment on Giant-Man, Wasp, and Hawkeye. Giant-Man became a full-on misogynistic Jerk Ass in the vein of his Ultimate Marvel counterpart; Wasp became a Shrinking VioletDomestic Abuse victim (oh, yeah, the Infamous Hitting Incident was brought back yet again for this just to cover all the bases, even though it had been supposedly finally laid to rest about three issues prior, during the Geoff Johns run); and Hawkeye was derailed into secretly having had a thing for Wasp and hated Giant-Man since the Eighties. All of this was promptly reversed by the next writer on the book, who killed Hawkeye and reverted Wasp back to her Sixties self before breaking her and Giant-Man up for good (or at least until someone else comes along).
As proof that no company is safe, Exhibit C for Chuck Austen is his short career with DC. After being handed the reins to the Superman book, Austen proceeded to make it clear that he hated Lois Lane's guts (especially in interviews), hated the Superman marriage, and preferred Lana Lang as Supes's one and only. His reasoning was that Lois Lane was a gold digging bitch who only married Clark because he's Superman (even though she agreed to marry Clark Kent, not Superman) and tried his best to make readers agree with him. Since his best "involved things" like derailing poor Lois into a huge shrew who didn't give a damn about her husband, and having Martha "Ma" Kent expressly make the point that she would have rather had Lana as her daughter-in-law, this went over like a lead balloon and ended with Austen being fired. Readers also rejected such retcons as Lana's reasoning for naming her son with Pete "Clark" (rather than it coming from respect for an old friend, Lana had picked the name because she was still hopelessly in love), seeing them as desperate and creepy rather than sympathetic. As with almost every other relationship Austen ever wrote, later writers either ignored or outright rejected his take on things.
Something similar happened to Black Cat in the Spider-Man comics. For publicity reasons it was decided to have Spider-man marry at the same time in the newspaper strip and the comic-book, unfortunately at the time Peter was dating Felicia Hardy/Black Cat. The solution? Make Felicia go from morally ambiguous cat-burglar to the ally of an assassin for hire.
Though that storyline was badly written and not helped by various production delays, this was no derailment. Peter and Felicia had broken up in 1984, about two years before the decision to have him and Mary Jane marry was taken. When the Black Cat started dating Spider-Man again - in Spectacular Spider-Man #123 (published a mere five months before Peter proposed to Mary Jane in Amazing Spider-Man #290), she was in cahoots with the Foreigner, the assassin responsible e. g. for the murder of Ned Leeds. It also was clear from the start that she was motivated by a desire for revenge against Peter for getting Dr. Strange to remove the bad-luck power she had bought from the Kingpin. And this period of dating began with the Black Cat faking a bomb attack on her own apartment to get a pretext to invite herself in as Peter's new flatmate.
This happens to a few characters during the Brand New Day run. Thanks to having a rotation of writers, each writer seemed to have their own OTP. Carlie Cooper however was the intended love interest, so that led to the derailing of several characters to make room for her. Michelle Gonzales, Pete's Tsundere roommate, suddenly became verbally and violently abusive towards him, and Mary Jane, despite still apparently being deeply in love with him, becomes more distant and even turns into a Shipper on Deck for him and Carlie.
However, once Dan Slott took over full time as writer, it took him less than a year to sink the Peter/Carlie relationship and brought Peter and Mary Jane much closer together as friends again
Happened during the 90's Howard Mackie/ John Byrne relaunch, where Mary Jane became more bitter and angry at Peter for being Spider-Man, to the point where she left him for a long period of time. Then she was blown up in a plane (she got better). All of this was done to get Peter single again, so he could start dating other characters, most notably Jill Stacy (Gwen Stacy's cousin.)
Strangers in Paradise: It was certainly very convenient that Francine's husband turned out to be having an affair near the end of the series, thus freeing her up to finally get together with Katchoo.
Devin Grayson is a female comics writer who gets to be in charge of a Teen Titans run. So far, so good. There's a problem, though... her OTP is Barbara/Dick, but he's with Koriand'r. What is Devin's solution? Bash the SHIT out of Kory by making her an absolute bitch to Dick, so he can run back into Babs's arms. Dick and Kory had been long broken up since the end of Wolfman's run, but Grayson's character derailment simply made things worse, especially since the two agreed that they could simply be very good friends at the end of The Technis Imperative (a mini-series co-plotted by Grayson and Phil Jimenez that lead to her run). Apparently, in Devin Grayson's view a male superhero can't remain good friends with his superpowered ex-girlfriend without Hell breaking loose...
Further complicated by the fact that when Devin Grayson was writing Nightwing's series, she'd clearly intended for him to propose to Barbara when she wrapped up her run. However, at the same time in Judd Winick's Outsiders, he portrayed Dick as having had a one-night stand with Kory to rekindle their relationship. The lack of communication between editorial departments further agitated fans of both pairing options, and had the result of turning Dick Grayson into a two-timer (which the character's reputation couldn't quite recover from for a while). A later Nightwing annual by Marc Andreyko further made the love triangle look worse, when it was retconned in that Dick had slept with Barbara the night before he was due to marry Kory. It was also revealed that in an earlier point of their relationship, Babs went to deliver him flowers but instead bumped into Kory (who he'd just had sex with as his "first time"). These revelations were meant to explain why the engagement plot point became nullified in the "One Year Later" timeskip. Instead, it just complicated the issue and only gave fans further impression that Dick was an unreliable and indecisive cheater.
Some writers at DC seem to adoreSuperman/Wonder Woman, as evidenced by the fact that many different Elseworlds either derail Lois Lane to get rid of the marriage, or simply kill her off entirely. A stand-out example is JLA: Act of God, where a mysterious event de-powers every non-technological superbeing on the planet, after which Lois breaks up with Clark because he lost his powers. Granted, the fact that Clark is busy being Angsty McWhineNCry about the whole thing really didn't help matters, but still.
And now, thanks to The New 52 reboot, Clark and Lois have never been married, thus allowing DC to finally hook Superman and Wonder Woman together. Of course, this is lampshaded in the final issue of Justice League International when a time travelling Booster Gold attempts to warn what's left of the team of something in the future, only to be interrupted by the two's first kiss and gawking in utter shock.
The infamous bitchslap that sent Sally Acorn into Jerk Sue territory has shades of this; after being a couple for several issues, a supposed mandate came down ordering Sonic to not be in a steady relationship with any female, and the next thing readers knew, Sally was bitching out Sonic for not ditching his duties to be her consort, which led to him getting into a torrid Love Triangle with Mina Mongoose and Fiona Fox. Incredibly, the mandate from SEGA did not prevent the publication of the "Moebius 25 Years Later" storyline and it's later spin-offs, which depict Sonic and Sally Happily Married with Babies Ever After !
Ian Flynn eventually put Sonic and Sally back together again in the main comics continuity...but only for one issue, as, following the "Genesis" storyline, it was revealed Sally had been robotisized, turning her against everybody.
Similar to Chuck Austin, Grant Morrison went this way with his X-Men run. Joe Quesada, being who he is, wanted to break up Cyclops and Jean, so he told Grant to kill her off. Perfectly normal in a super hero comic, but instead of just letting her die in a good, heroic way or even stuffing her in a fridge like most writers do, he decided the best way was to derail Jean, Scott, and Emma Frost in order to make a 'love triangle' before killing off Jean so Scott can get together with Emma. To do so, he had Cyke, currently in a dark place following his merge with Apocalypse, seek therapy to deal with his issues, which Emma provides with her previously-never-mentioned PHD in psychology. Then, she has psychic mind sex with him to let him live out his deepest sexual fantesies that Jean would never do so, while projecting the image of Jean. Jean at first is rightfully pissed at the two, especially Emma, but she recieves no punishment since she 'did it out of love' for Scott, despite the fact the two had never previously had much interaction beyong the psychic sex, and Jean is called out for having no sympathy for her. So, Jean forgives them all, only to be killed by Magneto despite being the Pheonix (and as such, immortal) at the time, with Jean then pushing Scott beyond the grave to move on. So in short, who was suposed to be 'in the right' was emotionally manipulating a man and is rewarded with becoming the second in comand, the real victim of the situation is made out to be a boring and dull prude, and the man in the middle is essentially mind raped when he needed therapy (which he never got, possibly explaining his own latentCharacter Derailment) and ends up looking like a tool and a douche. And that, ladies and gentlemen, is why some writers shouldn't write romance.
Brutally deconstructed in Young Adult. The main character Mavis (a whiny Girl-Child is convinced that she can seduce her ex-boyfriend to be with her again, despite the fact that he has a new kid. It turns out his wife feels sorry for Mavis and leaving her never entered her ex's mind.
Run Fat Boy Run: a guy enters the London marathon to win his ex back, who has a new guy already. New guy went from friendly Ace to a child-hating Control Freak. Granted, there were some early hints to his inner bastard - his competitive streak, apparent annoyance at children, and... he's American!
Subverted in Once Upon a Time in the Midlands; events seem to be going this way, with Dek (the new, sweet, rather hapless love interest) kicked out of the house and Shirley letting Jimmy (bad-boy ex and father of her daughter) move in and starting to fall back into a relationship with him. The subversion being that Jimmy is then revealed to genuinely be a dick who hasn't changed in the least, and Shirley does in fact get back together with Dek in the end.
In the film version of Harry Potter and the Half-Blood Prince, Ron's girlfriend doesn't do or say anything negative toward anyone until the very scene she and Ron break up. In the book, Lavender was a lot more clingy and annoying, to the point at which Ron pretended to be asleep so he wouldn't have to deal with her, leading her to start annoying Harry for a third party's views on their relationship. Of course, the real reason Ron was with her was because she was pretty and liked him; before they hook up, it's Hermione he's crushing on, not Lavender.
Used with Johann in The Hottie And The Nottie. Johann is introduced defending June from humiliation, and is a dentist, enabling him to clean up her crooked, ugly teeth. He's also a former Marine, a Harvard graduate, a great guitar player, a male model, a pilot, and other things that make him absurdly perfect, and much better than loser protagonist Nate. But when June eventually becomes beautiful and starts going out with him, he turns out to be more or less a self-involved jerk. However, the extent of his jerktitude is a couple of selfish comments that offend Nate and June.
Woman On Top falls on the "informed flaw" side of things. The ex-husband is truly sorry about cheating on her, but they needed to get rid of her new love interest. They get in a fight when he (under orders from the producers) attempts to change elements on her show. She dumps him over this single fight, which isn't even his idea, and goes back to the man who cheated on her just because he wasn't able to be on top while they had sex.
Literature
This happened in Twilight. When Edward leaves in New Moon, Bella gets close to Jacob, who falls in love with her. When Edward comes back, in order to justify Bella staying with him rather than Jacob, the author has to make Jacob do some kind of drastic, out-of-character things, like forcing Bella to kiss him, and threatening suicide to manipulate her. Then again, Bella already chose Edward as soon as he came back, before Jacob started to behave badly, so it's more of a "justifying her decision not to change her mind and go with the guy who treats her like an equal and actually seems to have any feelings for her besides claiming that he loves her" than a "justifying her decision to go back."
Of course, given how Edward behaves in the relationship, one could just as easily assume that Meyer was trying to make Jacob better as love interest material, especially since Bella only realizes she loves him afterthe forced kiss and all.
Many fans thought that the kiss in the eighth book as well as the circumstances surrounding his break up with Lily (i.e. the Anvilicious implications that he was only with her to remain close to Mia) as well as several other factors indicated that he was the Jerk Ass all along. Therefore, the events were not without foreshadowing, and so doesn't fit this trope. Of course, almost all JP/Mia shippers tended to overlook these moments in earlier books while Michael/Mia fans zeroes in on them.
Jonathan's behavior in the third book in Song of the Lioness series qualifies in spirit, if not all specifics. Up to that point, he'd been portrayed as a good, upright guy. He certainly was, in the second book, a bit focused on a girl but he was never condescending, crude, or such. Then in the span of one chapter he is that and more, all to push Alanna into the arms of the lovable rogue George and later Liam. Pierce had originally intended Jon and Alanna to be the Official Couple, but it was while writing the third book that she came to believe they didn't work together — and if Jon does come across as cruder, some of the seeds for his behavior were glimpsed in the previous book, it was just played out in a more romantic light. Because Jonathan believed they were "fated" to be together, he took her assent to things as all but a given, which was exactly what infuriated her later on when the things in question were marriage and family instead of sex.
The Author really did take it too far, though. It was spectacularly hard to like Jonathan after these scenes. It's not that there wasn't foreshadowing but more that his character was forcibly tossed off the rails just long enough to break them up, and then he just goes back to normal once she's firmly "not for him".
Erik from The House Of Night series is introduced and set up as a sweet, attentive boyfriend to Zoey with no real flaws in the first few books, and when they break up at the end of the third book, it's entirely her fault. Then he suddenly reveals a possessive streak in later books so that Zoey can be justified in showing more interest in Heath and Stark than him.
Perhaps the swiftest use of this trope is used in The Roman Mysteries. Throughout The Scribes of Alexandria Nubia spends months travelling to a Nubian settlement in the hopes that she'll find her own people there. She is eventually reunited with her betrothed, only to be torn between marrying him or returning to Rome with her friends. It's a tough decision, but luckily her betrothed goes from a nice guy to a potential wife-beater within the space of a paragraph.
Sweet Valley High and its spin-offs pulled this several times; usually to have the girls dump a one-shot love interest and return to their regular boyfriends, because Status Quo Is God.
Sisterhood series by Fern Michaels: As the series goes on, Isabelle Flanders forms a relationship with Stu Franklin, and Annie de Silva forms one with Little Fish. By the book Cross Roads, Stu and Fish end up suffering hugeCharacter Derailment! Both men start treating their girlfriends like prisoners, neglecting them, turn out to be cold-blooded murderers, and also show a huge amount of Ho Yay for their boss Henry "Hank" Jellicoe. Deja Vu tries to justify it by saying that people only let you see the best of them most of the time. Annie did hook up with Fergus Duffy and Isabelle did hook up with Abner Tookus, but they didn't even meet these guys until Deja Vu and Home Free!
In Pretty Little Liars this is actually Lampshaded when Noel broke up with Aria because of their vast differences, she emailed Ezra telling him she missed him and he came back, only to find much to her shock that he had become needy, insecure, and clingy, traits she noted that he never had or at least she hadn't noticed before.
Henry Crawford may be this in Mansfield Park, although it's hard to tell under all the Values Dissonance. His character in the first two thirds of the book is that of a moderately irreverent, idle rich boy, but the worst things he does - participate in staging a scandalous play, and mock another character for wanting to be a clergyman - aren't exactly high crimes by today's standards. In the last few chapters, after proposing to Fanny and having his suit rejected, he elopes with her cousin, bringing scandal on both families, and Fanny marries her nice-but-boring cousin instead.
Live Action TV
One is in the US version of Queer As Folk, where Justin leaves Brian for a much more romantic guy, who suddenly has an affair.
* Sex and the City's Alexandr is one of the best examples of this trope, going from an amazing, sweet, and sensitive guy to a jerk who ignores Carrie and proves to be completely self-absorbed before leaving for Paris.
Charlie in Ugly Betty went from sweet, likeable girl to jealous, two-timing harpy in no time. Then again, the show is partly a soap parody...
Totally out of the blue, Tess on Roswell was revealed to have been repeatedly brainwashing their friend and conspiring with their enemies so they could break up her and Max.
Played perfectly straight in the BBC's Robin Hood. Robin starts a relationship with Isabella, a seemingly intelligent, rational, and compassionate woman. However, halfway through the series, Isabella throws an inexplicable temper tantrum, goes insane and tries to kill him after Robin tells her that his mission must come before her. This is apparently done so that the writers can kick-start Robin's love story with Kate, the Betty to Isabella's Veronica. The problem was that Kate was the most reviled character in the entire series, generally considered to be brainless, whiny and often downright spiteful, whereas Isabella (the victim of an abusive husband and heavily implied to have mental problems) was mostly sympathetic. She also had excellent chemistry with Robin, whereas Robin had more chemistry with his bow than he had with Kate. Oh, and if you're wondering where Maid Marian is during all of this, she was murdered by Isabella's brother last season. Yeah...Season 3 sucked.
In Friends, Ross's English fiancée Emily was subjected to this when the actress got pregnant and had to leave the show.
After Chandler moved in with Monica, Joey's new roommate was a woman to whom he was attracted and eventually got together with. Then, she inexplicably turned out to be unable to stand Chandler and Monica, causing Joey to break up with her and her moving out.
This trope is used in many other instances on the show as well.
In Family Matters, Myra's Clingy Jealous Girl behavior towards Steve is turned Up to Eleven (Even after they break up) in the final season to the point where she installed a spy cam in his glasses so she could watch him through a monitor in her bedroom. This so Steve, who'd been happily in a relationship with Myra for years, could be paired with Laura (whom he'd had a completely unrequited crush on, played for comedy, for a good while, but by now, had long been dropped.)
In Home and Away, this happens to Sam Tolhurst. At first she comes across as a sympathetic character, with a tragic backstory, (on the run from accomplices of her drug dealer husband) and is portrayed as a good match for Jack. Unfortunately, when they get engaged, and especially after getting married, she becomes a possessive controlling bitch, especially towards Jack's ex-wife Martha.
At first this is justified (specifically when she walks in on Martha wearing her wedding dress), but the writers derail her during her exit storyline. She murders a criminal who had been blackmailing her to force her to hide him, after which we learn that she had lied about her backstory — she was the ringleader in her husband's drug operation, and was on the run because she'd allowed him to take the fall.
After she's exposed, she kills herself and almost succeeds in framing Jack and Martha. The worst part is that the writers left several details out of this retcon, (the fact that she'd tolerated Shane's drug dealing up until she'd witnessed him killing someone in cold blood, and the fact that he'd been doing it to raise money for their terminally ill daughter).
While it would have been entirely plausible by this point to handwave these as Sam's lies to Jack, they are unmentioned in the letter which reveals the truth, and Jack himself doesn't bring them up when he confronts her about it.
With Genre Savvy resignation, Juliet from Lost decides her three-year relationship with Sawyer is over as soon as Kate returns. She is sure Kate is the one for him, heedless of Kate's or Sawyer's opinion on the matter. In a subversion, they reunite in the afterlife while Kate is with Jack.
Repeatedly done on The Drew Carey Show as part of the central character's general run of bad luck. Nicki left him after gaining a considerable amount of weight. Working class Wrench Wench Sharon seemed perfect for Drew, but it was later revealed that Drew was in fact the other man when they meet her real boyfriend at a ballgame. The intended endpoint was a long-term thing with Kate, who was ironically derailed herself when Christa Miller ditched the show for Scrubs.
When Drew started taking night classes, he met an older woman, and they started dating. There's a lot of fun at their expense over the age difference, but they seem like a happy couple. But then in her last appearance, she begins mothering Drew (when previously she had treated him like an adult). It got to the point that he couldn't stand it anymore, and they broke up.
Louis in Gossip Girl has been steadily becoming less sympathetic over the course of Season 5, as Chuck becomes a better person. This is lampshaded by Blair, as well as explained by the fact that Louis is well aware that in a competition against Chuck he'll lose no matter how nice and sweet he is, prompting him to become more underhanded in his desperation to keep Blair.
In the episode "The Grinning Man" of Jonathan Creek, Jonathan manages to land himself a pretty, intelligent and caring girlfriend who (despite a slightly ditzy interest in magazine quizzes), is refreshingly normal compared to many of Jonathan's other love interests and is understandably worried when he goes investigating murders. At the end of the episode she runs off with Joey's boyfriend despite having never met him, paving the way for a possible Jonathan/Joey relationship.
iCarly used this in an episode, to break up Jonah and Sam. They spent 60% of the episode being sickeningly sweet to each other. All of Sam's attention was being spent on him, and it was causing problems for her friendships with Carly and Freddie. Also impacting on their webshow. Instead of Sam coming to realize that she can't spend all her time with him, or have Jonah realize the same and back off slightly, he suddenly tries to cheat on Sam with Carly, making all the potential issues moot when they wedgie-bounce the sucker for trying to cheat.
Which actually made sense after iKiss with a bit of WMG. In iKiss, which happened after she dated Jonah, Sam says that she's never kissed anyone, meaning that Sam must have never kissed Jonah. It's possible that Jonah got fed up with it and tried to kiss Carly instead.
In Freaks and Geeks, Sam is hopelessly smitten with cute cheerleader Cindy Sanders. Toward the end of the show's first and only season, Cindy finally starts dating Sam, and they almost immediately learn that they have no common interests and don't have any fun together. The previously sweet-natured Cindy is also revealed to be (mildly) bitchy and manipulative, as well as a bit vapid and dull.
Some argued that the third season finale of The Office (US version) gave this treatment to Karen. After Pam revealed she called off her wedding because of Jim, Karen, having served a whole season as a likable match for him (sparking Team Karen and Team Pam factions in the fandom), the character was shown pushing Jim into promising to move to New York with her if either got promoted to the open corporate job there (explicitly stating that she wanted to get him away from a certain other person), and leaving to have lunch with friends while he had his interview. Others questioned how the show could arguably portray her as a villain for these actions but have Jim abandon her in New York, after they drove up together, just so he could drive back home and ask Pam out right away (though he acknowledges his behavior in the season four premiere).
Possible subversion can be seen with Erin and Gabe. While the couple was portrayed as normal and seemingly happy at the beginning of their relationship, they soon began to show conflicts between the characters leading up to Erin's harsh break up with him. However, the conflicts in their relationship were based on character traits that were already established, and played straight/exacerbated with a few new ones.
Subverted in favor of the dramatic route on Battlestar Galactica, where Kara and Anders' relationship stays intact despite Kara's affair with Lee (Kara and Lee having been the Fan Preferred Couple in the previous season).
Dean's wife Lindsay became a shrill harpy who overworks her husband as Rory suddenly becomes interested in Dean again (although Lorelai called her out on this one). Ironically, Dean would get derailed himself for the actor's exit from the show.
Jason threatens to sue Richard after he and Jason's father essentially destroy his livelihood, just in time for the Luke/Lorelai relationship to finally happen.
Marty is at first a sweet classmate who takes an interest in Rory but she spurns his advances and he takes a long bus ride only to return as a huge Jerkass in the last season.
Arguably happens to Charlotte in H 2 O Just Add Water. She starts off the season as a good natured girl who is able to be there for Lewis when Cleo is dealing with her Wangst. Suddenly halfway through the season she becomes jealous and manipulative, suddenly obsessed with discovering the girls' secret. When she becomes a mermaid she becomes a full blown villain thus making it okay for Lewis to break up with her so he can end up with Cleo.
Frequently used on Soap Operas, in scenarios that play out almost exactly as in the page description. A formerly decent third character is almost inevitably destroyed in order to prop up either the show's designated supercouple, or whatever couple the writers wanted to pair up.. A particularly good example is on As the World Turns, where Jack dumped the manipulative, scheming Carly to hook up with Mary Sue Julia. Fans clamored for a reunification of Jack and Carly, so within a year, Julia went from Mary Sue to Yandere, pulling increasingly horrific stunts that included attempted murder, kidnapping, and RAPE, giving Jack the perfect excuse to return to Carly—with whom he proceeded to have an on-again, off-again relationship that was just as unhealthy and destructive as their previous one (and often included the sacrifice of even MORE Derailing Love Interests for both of them) before finally marrying for good in the series finale. An almost identical example played out on Guiding Light with Josh, Reva, and Annie.
An interesting subversion on General Hospital, where rather than being an obstacle, the Derailing Love Interest was part of the Fan Preferred Couple—the male half of the pairing was already in a happy relationship, and of course, it would make no sense for him to break up with a woman whom he adored. But rather than inexplicably ruining her character, the writers instead had him develop complete amnesia following an accident, leaving him with no memory of or regard for her, thus paving the way for his new relationship.
Played completely straight in Glee: After Rachel and Finn's sort-of relationship hits a rut, she meets Jesse St. James, the lead singer of rival Glee club Vocal Adrenaline. Naturally he's her equal, understands her perfectly, etcetera. When he is very obviously The Mole, he admits that he's starting to have feelings for her. Right after that happens, of course, he becomes utterly cruel with the egg-assault in the parking lot and him acting like she somehow did something to deserve it, especially since the last time we saw them they were perfectly happy together and she had done nothing to upset him. It's clear he's only doing it because he wants to go back to Vocal Adrenaline so that he can win a fourth national show-choir championship, and needs to prove his loyalty to the team above all else. But it may be subverted. The Bus Came Back with him on board, and he ends up apologizing to Rachel and is obviously still very much in love with her. However, she didn't care and got back with Finn
Coach Beiste's husband is randomly revealed to be abusive. Granted, Dot-Marie Jones sold the storyline for everything she was worth and then some, but it came completely out of left field.
Also with Quinn in Season 2. When Finn and Quinn got back together in Season 2, their relationship was very believable and well-received. However since the writers obviously wanted Finn and Rachel back together, they make Quinn controlling. (Of course Quinn has also been derailed when she wasn't a love interest)
This was forced on Will Griggs on Neighbours, not because he was a Romantic False Lead, but because the actor had left the show early and they chose to bring in a Suspiciously Similar Substitute. At the time he was written out, Carmella was furious with Will for lying to her and hiding his real identity as Sebastian Barnes one of the richest men in the country, but he was still trying to get her to forgive him. Then suddenly he was gone, and his adoptive brother Oliver arrived to reveal that Sebastian's entire personality had been faked, and was based on Oliver. This led to Carmella and Oliver getting together, and apparently rendered insignificant Will's substantial number of good deeds, such as standing up to Paul's attempts at blackmail (his 2% share of Lassiters was enough to give Paul a controlling interest, and Paul used that to get Will's friend Ned fired); standing by Carmella when she was badly burned by her mentally unstable cousin, and thwarting an attempt by Janae's pervert of a boss to spike her orange juice. It got worse later in the year when Oliver's biological brother Declan was kidnapped, and Sebastian (offscreen) only agreed to pay the ransom if Oliver gave up his share in the Barnes hotel chain.
Happened to Eddie Le Bec, Carla's husband, on Cheers. They foreshadowed it for a few episodes, but transformed him from the nicest guy you'd ever want to meet, to a cheater who married the woman he had his affair with and never told Carla (it became public at his funeral). This was deliberate payback on part of the producers for Jay Thomas having insulted Rhea Perlman's attractiveness on national radio.
The writers of Frasier saw this trope looming at them from a mile away and nipped it in the bud by never bothering to make Niles' Romantic False Lead Mel Karnofski into a sympathetic character in the first place. From the very start she was both characterized and described as a slightly-kinder doppelganger of Niles' first wife Maris, a controlling, domineering, hysterical Manipulative Bastard and Jerk Ass extraordinaire (whom all the characters despised), explaining Niles' attraction to her as a bad pattern (similar to how some women who leave an abusive boyfriend fall for men exactly like him over and over), instead of wasting time trying to make her into a person whom Niles could fall for for healthy reasons, but the fans would inevitably vilify anyway.
Supernatural has an odd case of a love interest being derailed for reasons unrelated to their being a love interest. In season 4, Dean meets Anna Milton, a fallen angel whom he quickly takes a romantic attraction to. Originally, she was going to replace Castiel (who was going to be killed by Alistair) as Dean's angel guide, however this hit a snag when Castiel became the show's Ensemble Darkhorse, with both fans and critics responding more positively to him than any character other than the Winchesters themselves. Thus, Castiel was spared and his role in the series expanded. This had the unfortunate side effect of making Anna redundant, however, and she was given only a few more appearances before Jumping Off the Slippery Slope and deciding she needed to erase Dean and his brother Sam from existence.
Smallville made a habit of this with Lana's love interests. Jason Teague was thrown into the Title Sequence in his very first appearance, only to get less and less screentime until, at the end of the season, he was revealed as a member of some ridiculous secret society and then unceremoniously killed... by meteors. This was only one season after Lana's previous boyfriend, Adam, became increasingly unstable and villainous... oh, yeah, and he was dead; the villainous version was her shape-shiftingStalker with a Crush.
Charmed actually derailed both lovers in the relationship at different times with Phoebe and Cole. The relationship went through a LOT of drama ( Cole became possessed by the Source of all evil, made Phoebe his queen and got her pregnant with a demonic baby which forced her to kill him). She ended season 4 still loving him but just afraid to use magic to resurrect him. Then the first episode of season 5 she suddenly blames him for everything that's gone wrong in her life and threatens to kill him over a bad dream she had. The writers then realised Cole was looking too sympathetic and abruptly made him go mad and evil and thus had to be killed off permanently. It still made Phoebe look like a Designated Hero though.
Happens to Nick in How I Met Your Mother, who was originally depicted as a hot guy who was both mature and had subtle snarker tendencies. But later in season eight, he becomes "girly", overly sensitive, has Granola Guy tastes and is Too Dumb to Live. There was attempted justification that he was an idiot all along but was too hot for Robin to notice that.
In the same episode that it was "revealed" Nick was overly sensitive, it was "revealed" that Victoria was a total slob at home, something that has never been hinted in her previous appearances and given her occupation as a highly skilled pastry chef, it seemed rather unlikely. Thankfully, it was averted in the episode that she and Ted broke up where her being a slob wasn't what caused them to break up and Lily acknowledged that Victoria had valid reasons for breaking up with Ted because of Robin.
Newspaper Comics
Towards the final years of For Better or for Worse, any non-Anthony entity that Elizabeth showed any romantic interest in, no matter how sweet and kind, were eventually shown to be cheating on her, stalking her, or otherwise completely disrespecting her. The record being her date to a wedding reception getting drunk and falling asleep the same night. Note that Anthony did in fact stalk Elizabeth and emotionally cheated on his wife, but these were brushed aside because he was Anthony.
The dumbest break-up was with the helicopter pilot. First Liz mistook his sister for a secret girlfriend, then he said he couldn't be tied down.
Speaking of Anthony's wife: Similar to the Fraiser example Theresse had never been portrayed in a good light. She's jealous of Liz at her own wedding, is very ambitious, has no desire to be a mother (the most unforgivable sin in the Foobiverse), had Anthony do all the childcare, and in her final appearance abandons her child forever. Then again there's the fact that Anthony pretty much forced her to have a child and ignored what sure looked like post-partum depression.
Videogames
The game Fire Emblem: Seisen no Keifu has two characters, Eltoshan and Lachesis, who give huge Brother-Sister Incest vibes despite Elto being married to a lady named Grahnye and having a kid with her, Aless (who becomes a playable chara in the second part of the game). One of the artists hired to make manga adaptations of the game, Mitsuki Oosawa, is an Eltoshan/Lachesis shipper. What doe she do? Aside of erasing one of Lachesis's prospect love interests, Beowulf, she writes Grahnye out of the story... via introducing an Expy of her named Iria and making her a HUGE Rich Bitch who clings to Eltoshan and hates Lachesis to death, while cranking up the canon incestuous subtext Up to Eleven. This gets so bad that Iria gives Lachesis permission to join Sigurd's group solely to get her killed in battle, and when Eltoshan is executed, she barely shows any pain for it, and quietly gloats about how unhappy Lachesis must be.
Western Animation
The Simpsons used this in an episode where, before they were married, Marge goes to college and leaves Homer for a Strawman Liberal professor who is smart, worldly, and seemingly more attentive to her, while he actively sets out to make Homer's honest love and devotion look like pig-headed, Stay in the Kitchenmisogyny (which could itself be seen as invoking this trope). Once they start dating, the professor suddenly becomes condescending, passionless, and passive-aggressively misogynist, the final straw being that he's anti-marriage. He'd been a pretty insufferable guy up to that point, but became significantly worse once he'd got rid of the competition. Homer in fact lampshades it at the end; "don't worry, kids, she'd never have stayed with that professor... unless he'd been a little nicer to her at the beach!"
Total Drama Island had a sort of version of this in the second season. Trent became jealous of Duncan and Gwen's friendship, which makes some sense, but also became a ridiculously-clingy stalker and, weirdest of all, out of nowhere developed a supposedly-longstanding obsessive-compulsive fixation with the number nine. Seriously. Different than many of these examples because Gwen did not immediately run into another man's arms, though the show keeps teasing the idea of her and Duncan getting in a love triangle with Duncan's girlfriend Courtney. Basically, the whole situation was badly-done.
Add Courtney's Character Derailment in Total Drama Action at the list, as well...
The Love Triangle between Duncan/Gwen/Courtney is a fact in Total Drama World Tour...and eventually Duncan cheats on Courtney for Gwen (destroying their budding friendship) with no explanation or motive, with Courtney receiving further derailment (and arguably derailing Duncan and Gwen's personalities as well). Enough for Heather to compare Courtney to crazy Izzy.
Averted/subverted/played with in X-Men: Evolution after the mutant revalation and the series cut up most of its romantic plot threads:
Duncan and Jean broke up because he acted like a jerk. However, he always was a jerk and Jean had expressed a lack of interest in the relationship any more beforehand because of his jerkitude. He totally crossed the line when began attacking mutants with guns and bombs!
Scott and Taryn broke up in a slightly straighter example as while she was always nice, she aparently no longer wants anything to do with him, but then again she just found out he's a mutant who could accidentally smash planets with his eyes, and who knows what could've happened if given the chance to see beyong that.
Family Guy: Quagmire gets engaged to a calm and attractive maid. Throughout the episode, the relationship was loving and Quagmire even lost interest in other women. Peter and the gang's attempts to get Quagmire back to his old womanizing ways fail, up until a fancy dinner, where Peter's actions with Lois remind Quagmire of what he really wants. Then his cheerful patient bride to be is revealed to be a self-destructive Yandere out of nowhere, therefore absolving Quagmire of getting out of the relationship. Status Quo Is God in this show, so the relationship was unlikely to survive the episode.
