Mamoru Chiba/Tuxedo Kamen/Prince Endymion of Sailor Moon is definitely up to his neck in this trope. The guy is consistently portrayed throughout every incarnation of the Sailor Moon franchise as Usagi's love interest, but The '90s final anime series, Sailor Stars, introduced Seiya Kou as a surrogate love interest while Mamoru was absent and fans, seeing a het-ish alternative to Mamoru ("ish" because Seiya is a Gender Bender in the anime and a Sweet Polly Oliver in the manga), proceeded to take this opportunity to express their anger at Mamoru. Said anger stems mostly from a plot arc in the R season, where Mamoru broke up with Usagi after receiving disturbing dreams that their relationship would lead to The End of the World as We Know It. It really didn't help that the whole arc reeked of Idiot Plot in general and the entire situation came off as very contrived and melodramatic, not to mention being incredibly dragged out. It also didn't do any favors for Mamoru's character either, as his repeated attempts to push Usagi away came across as mean, even though he did have good intentions. But what people tend to forget though is that after that issue was finally resolved, Usagi did forgive Mamoru and he generally went back to being a serious and overall devoted and well-meaning boyfriend. But Never Live It Down is in full effect here, as some Seiya/Usagi shippers will practically bend over backwards to color Mamoru up as this heartless asshole who hates her.
There's a Squicky subgroup that claims that Mamoru is in love with his own future daughter. Anime Usagi is shown to be so childishly jealous when Mamoru pays attention to Chibi-Usa that she even asks him in the Super S movie which one he loves more. Mamoru is understandably flabbergasted and gives no response, which sets up a Chekhov's Gun when Usagi is later trapped in an illusion with a fake Mamoru. Some of Mamoru's detractors will use this as evidence that he loves his daughter more than his girlfriend/future wife. It does not help that Chibi-Usa is quite clingy to him as well, and even refers to her father as her boyfriend. Still, this subgroup seems to be unaware of the time in R when the Black Moon clan tries to brainwash Usagi into joining them by attempting to make her think that Mamoru was in love with Black Lady, Chibi-Usa's magically aged up and brainwashed and evil alter ego... but Usagi breaks through it and says this won't happen ever. Note that in the manga, Usagi's jealousy of Mamoru's attention to Chibi-Usa was actually addressed directly when Usagi confessed it to him - leading to Their First Time. Needless to say, Mamoru was a bit more direct about his feelings in the manga.
This also happens in reverse. Some Mamoru/Usagi fans don't take kindly to people splitting up the "Ultimate OTP" of the fandom. As a result, Seiya also gets a lot of hatefics where she's treated as if she deliberately tried to ruin Usagi's relationship to steal her from Mamoru, even though in canon, she respected Usagi's prior commitments, and in the anime, mistakenly thought that Mamoru had dumped Usagi without telling her. ( Mamoru's death wasn't revealed until the end of the anime's plot.) Plus Seiya did gracefully let her go after realizing she wouldn't ever be truly happy with her. Even Mamoru trusted her and Usagi and was on pretty friendly terms with Seiya when they finally met. Both versions of Seiya (the Gender Bender from the anime and the Sweet Polly Oliver from the manga) took heat from the rabid shippers; the first more than the second, yeah, but still.
The infamous "Am I not enough?" scene and Seiya's cocky attitude, while valid flaws to be noted, tend to get twisted by some of the anti-Seiya crowd into proof that Seiya is a manipulative asshole that wanted to ruin Usagi's well-being if it meant she could have her. And lately, the poor man/woman is being derided as a creepy sexual predator for daring to flirt with Usagi at all. While s/he did come on too strong in the anime, s/he hardly shoved her against a wall and tore at her clothes the way some fans would have you believe.
And then there are the Mary Sue fics that bash Usagi to have Mamoru get it on with the Mary Sue. Or the crossovers that bash not only Mamoru, but also all of the Senshi (specially Rei, who briefly dated Mamoru in the anime but gave him up willingly) so Usagi will ditch the team and get together with Ranma Saotome, Farfarello, Heero Yuy, or Trunks. Or the Usagi/Rei yuri fics that sound very similar to the Seiya/Usagi ones that make Mamoru a Jerk Ass...
All you have to do to split up Usagi and Mamoru is to make Mamoru a Jerk Ass. One Usagi/Haruka fic, without irony, condemned Mamoru for being a possessive, unloving jerk who was torturing Usagi's emotions and driving her into Haruka's virtuous arms. Meanwhile, it seemed approving of Haruka's romance with Usagi while ignoring her live-in lover, Michiru, who suffers extreme Chickification and is okay with Haruka cheating on her (!). The story ended with Usagi and Mamoru together after Mamoru was harshly scolded for his evil ways and Haruka nobly giving up Usagi and still getting to walk home with an ever-loving Michiru.
ReiHino was demonized quite a bit by rabid Usagi/Mamoru fans for her crush on Mamoru. They painted her as a screaming, frothing home-wrecker who wanted to kill Usagi and steal Mamoru all for herself. (The English dub making Raye a flat-out bully towards Serena didn't help matters.) Never mind that in anime canon, Rei immediately stepped out of the way once she realized Usagi and Mamoru were meant to be, that she was the most protective of Usagi among the Senshi, that Usagi trusted her the most out of anyone in the Senshi, that she was somewhat paired up with Yuuichiro, and that Rei was the most affected in the Stars anime when she learned that Usagi lost contact with Mamoru and hid that from them. "Why must Usagi suffer so much? It... it's so unfair!", anyone?.
Don't forget the Super S episode in which Mamoru had to spend the night at Hikawa since his apartment was in repairs. You'd swear it's a Take That! to the Rei-bashing-from-Usagi/Mamoru-rabidshippers as Usagi got comically jealous of Rei and recalled how she used to chase after him, only for a squicked Rei to go all "What the HELL are you smoking?!"
It was once popular, during the English dub's heyday, to portray Haruka as a Psycho Lesbian who either perverted or abused Michiru, all to ensure that a Relationship Gary Stu could enter the picture and rescue her from the sin of homosexuality. A subversion occurred on a once-infamous website that posited the reverse - that Haruka was a shy, gentle heterosexual woman and Michiru forced her to be a manly lesbian. These days, now that Ho Yay is far more popular, they're one of the series' most popular pairings.
Even minor characters are not immune to this. Reika in particular is not popular among shippers just because she's Motoki's girlfriend. A lot of fans prefer to pair Motoki with one of the Senshi, some of whom have crushes on him. Said fans were probably thrilled that Reika got Adapted Out of Pretty Guardian Sailor Moon and that Motoki and Makoto became a couple in that series.
Another minor character who sometimes gets this is Umino, who was a Second Love for Naru after Nephrite died. Viewers found the Naru/Nephrite storyline so sweet and memorable that more than a few shippers would rather find a way to bring Nephrite back and have him with Naru. It doesn't help that many fans find Umino as annoying as his friends do. The guy's lucky that Makoto/Nephrite is so popular and that Naru/Umino is an Official Couple, or he might not get shipped at all.
Speaking of the Dark Kingdom Generals, Queen Beryl used to come under a lot of fire from Kunzite/Zoisite shippers. The fact that she killed Zoisite for his disobedience obviously meant she had some sort of vendetta against the relationship on the whole, and some fanfics would even cast her as The Vamp bent on seducing Kunzite and breaking up him and Zoisite. Granted, Zoisite's extreme jealousy makes him mentally casting her in such a role slightly plausible, but still.