Advice and Trust: In chapter 6 Rei shares details of past battles with a classmate... but she accidentally phrases it in such a way it sounds like if she is describing the pilots' sex life:
Rei:"Since after the Fifth Angel, when he forced open my armor with his blade and burst into my Entry Plug. No one else had ever gotten to my core with such need. He was urgent and forceful. It... made me smile." The faintest hints of such an expression appeared on Rei's lips. "I would like to do it again. Maybe more than once."
In the NarutoPeggy Sue fanfic "Again", Haku is described as being an innuendo magnet since any conversation or situation that Haku is part of almost invariably sounds innuendo-filled to everyone else, and it appears to be contagious...
In chapter 2 Misato tells Ritsuko Asuka has fainted and Shinji is examining her… and Ritsuko asks what that examination consists of:
Misato:"Shinji is examining her! He says she has a high temperature!" Ritsuko:"He's examining her? What kind of 'examination' is he giving her now?" Misato:"Cut that out! I'M SERIOUS HERE!!!"
In chapter 5 Asuka and Shinji are playing an arcade game and Asuka’s character is going underwater:
Asuka (seductively):"Oh...yeah, Shinji... yesssss... deeper... I... I..." (suddenly furious:) "I'M GONNA KILL YOU!!!!" Shinji:"Hey, it's not my fault if I always find the torpedo launcher when you're underwater!"
Clop It: After Celestia has given the group 'Royal Re-Sexualization Gadgets' we get this simply marvelous exchange.
Twilight: So why did you give me one?
Celestia': Twilight, your friends are all fags. It’s only a matter of time until they try to turn you too, and you’ll need something to defend yourself.
Twilight: So what should I do... use it to try to beat them off? Of me?
Curse of the Demon Pony: Early on Twilight remarks that she set up a few booby traps in her room to keep her brother out of her "bits". This sounds reallycreepy, even if you're aware of the fact that "bits" means "money" in the show.
Dave Stdider Pokemon Traner: Chapter 13's title, "STEAMY SEX! WITH GOSTBUSTERS!" is an example of this. Yes, this chapter has a sex scene. No, the Ghostbusters are NOT INVOLVED WITH IT. They're just in the chapter. That's what the "WITH GOSTBUSTERS" means. They are not being sexed up in this chapter.
Misato:"The rookie did good, […] In fact I think he did better than your first time, Asuka." Asuka:"My first time was at the age of seven, […] Besides, it's all just point and squirt; you'd expect a boy to be capable of that by his age. […] Ugh! Please don't ever quote that out of context. Or at all, in fact."
In Fishsticks, Feferi tells Eridan that she both loves and hates him. Feferi is referring to their dysfunctional relationship, but Eridan thinks she's quadrant flipping.
Game Theory (Fan Fic): Yuuno fiddled with his Device in both hands, and sighed. ... It felt wrong in his hands. Eek!
The author of Hidden Prophices didn't know what the word rape meant until after she used it.
Higher Learning: When Misato explains Ritsuko that Shinji and Asuka are practicing for a concert, she uses sentences such like "They're at it every night" or "Sometimes they ask me to join in". As a result, Ritsuko completely misunderstands what Misato is talking about.
From Moon Daughter, quite a few involving a misspelling of "horse":
"Some wild hores ran over the hill."
"It ran away and I chased it faster, it was a thrill chase, that hore would be mine!"
"Do you notice something weird about these hores," Said Albaster suspiciously.
"This is a secret hore of Artemis" I diclared.
An In-Universe example takes place in Movie Night At Freddy's. In the movie within a fanfic, Chica sings a song about her love for pizza. While it was intended to be cute and endearing, the movie plays it as a sexual fetish for pizza. She even fantasizes about marrying a man with a pizza for a head in the song. As a result, Mike and Bonnie are both squicked out by the song and subsequent references to it.
Whether it's actually accidental depends on whether My Immortal actually is a parody or not, but regular words are occasionally "accidentally" spelled wrong and turned into obscenities, e.g. "Minister of Magic Cornelius Fudge" becomes "Mystery of Magic Cornelia Fuck", "Professor Slughorn" becoming "Professor Slutborn", etc.
"Breasts" are often used unintentionally, in many different contexts. In The Evil Gods Part 1, the author talks about corpses being eaten by "foul breasts" (beasts). In Threat of Satanic Commonism, the heroes meet with the "head breast" (priest) of Moscow, and in The Titans Strike Back, Draco and his mob kill some "false breasts" who try to stop him from killing the British royal family.
In The Evil Gods Part 2, our heroes go to "Washing Dick", and get some "boobs" from witches.
In The Titans Strike Back, we hear it's a co-story with Attack of the Spank (Sphinx).
The summary of the Harry Potter fic Rushing by maxfaith reads thusly: "The night before Ginny leaves for her last year at Hogwarts, Harry learns that happiness is no longer beyond his reach, but held tightly in his hand." It's Ginny's hand, not his penis.
RWBY: Reckoning: Darrel's explanation when he's caught sneaking out of the medical wing is that he wants to "go and sleep with RWBY".note If you're not familiar with the original series, the leader of said team is named Ruby. The team name is pronounced Ruby.
The So Bad, It's GoodRedwall fanfic Soulless Shell is loaded with this. Perhaps worst is the bit where Leif attacks a rapist with the words "Now feel the pain that so many innocent young girls like her have felt but with oh so much more intensity". Disregarding his impressive lung capacity, what the hell is he doing with those magical knives?
Then again, see the Literature folder for why that's about the only part of the fanfic that's reasonably authentic.Also see the MST:
Romsca: (shuddering) I've 'ad just about enough o' this. Does this author even know wot 'e's sayin?
Lyulf: Lessee wot we got so far... 'e seemed like a paedophile, 'e's already in bed wid 'is mum, 'e did unspeakable things ter Arnold, Rajh is his secret gay lover...
Ara: ...and now he's necrophilic towards his dad. Lovely. I don't think even Freud could have thought that one up.
In Star Kits Prophecy, after it is remarked that it is StarGleam's second time out hunting, we get this gem: "she'd had her first time when she was a littlekit and she and FireStar had snuck out together that's why he chose her as his apprentice_." Squick.
The female alien in With Strings Attached is named “Shag.” Word of God states that the author didn't know what the word meant in British English until years after she first named the character. After she found out, and once she decided to finish Strings, she decided to leave it as is and see if anyone noticed.
In the Pokémon fanfic Pokemon Take Two, after Alex asks Ry and Raion if he can join them on their journey, we get this little gem from Ry (after he hug-tackles Alex): -> Ry: That'd be AWESOME! Just think of all the fun things we could do!
The title of Episode 67 of Sonic X: Dark Chaos is "The Fudge-Covered Trap." Considering the chapter features male-on-male child rape, well…