Precision F-Strike/Fan Works

The Ranma ½/Teen Titans crossover duology Dark Titans uses a Precision F-Strike as a Crowning Moment of Awesome; near the end of The Titans and the Lost Boy, Brother Blood has a laser blade to Kasumi's neck, and has ordered the Nerima Wrecking Crew and his rebellious students -- including some of the Nerima regulars, themselves recruited earlier in the story -- to kill the Titans or else. Both sides appear to be hopelessly stuck... until Kasumi quietly calls Blood... something... and shocks him so badly that she's able to break his grip and judo throw him.
My Immortal: "WHAT THE HELL ARE YOU DOING YOU MOTHERFUKERS!" Shocking because it's the Dumbeldore's Incoming Ham moment.
Yea verily, though I charge through the valley of the shadow of death, I shall fear no evil, for I am driving a house-sized mass of "fuck you."
"Look me in the eyes, Light damn you!"
Pulled off to great effect by Hinata in Team 8: "That's bullshit!" (pause, slaps hands over mouth)
"DON'T CALL ME PIT, DAMMIT!"
Finishing the Fight MC vs Matron Baenre. During the assault on Mithril Hall, Baenre Teleports out. The MC's response? "Shit!"
Those Lacking Spines, about entering Voreland: "ABSOLUTELY FUCKING NOT!"
If you can get the DC Nation universe version of Connor Hawke to the point of swear words, it's usually one of those.
For the most part, the Shocker fan fic Polarity avoids any swearing worse than "damn" or "crap." Even after fighting zombies, avoiding heroes, brawling with villains, and getting his ass handed to him night in and night out, Herman Schultz and company keep the language tame...until Big Bad the Grim Reaper shows Shocker what happened to the last guy who turned him down, revealing the Punisher trussed up like a stuck pig slowly being bled to death.

After my jaw dropped for a few seconds, I decided it was time to sum up this entire situation in a very simple and succinct phase. "You're fucking nuts."

In Pretty Cure Heavy Metal episode 24, Shugo fakes an Australian accent and lands a precision N strike in a parody of the N word privileges supposedly held by white Australians. The rest of the N words are spoken by the token black, who has all the N words in episode 25.
- Later, in episode 51, Shugo drops an uncensored F bomb right before she dies from her wounds, just to maximize the impact. She gets better.
In Kira Is Justice: "Oh crap." Bear in mind that the author almost never uses any curse words in his writing. Not even "stupid".
In Relationships Series story "Pain", Shamal asks "what the hell happened" to a badly injured Yuuno. Vita notices that this is the only time she has heard Shamal swear.
Takeru Takaishi from the Tamers Forever Series delivers one of these:

"Do you understand now? We're not going to a fucking walk in the park. This is not a goddamned picnic."

Elemental Chess Trilogy: "You leave my family alone, you cowardly son of a bitch!" It's a Crowning Moment of Awesome for the speaker, and an odd sort of Crowning Moment of Heartwarming since the "family" in question are the speaker's True Companions. It's also the only time in the entire series that anyone uses a swear word other than "damn."
Harry Potter: Draco nails this trope in Yea, Though I Walk when he explains to Hermione, that killing Snape won't stop the demon that is claiming the Death Eaters from possessing his body The cursing and Draco's grief due to mercy killing his possessed father in vain, and her mother sacrificing herself to the demon to save her son really highlight that the situation seems completely hopeless.

Draco: You can't kill a fucking demon with a pillow!

- In another Harry Potter fan fic straight-laced Percy realizes that the Minister of Magic is under the Imperius curse, which could only mean Voldemort is controlling him. His mental response: "Oh, shit."
The Gift: While other characters seem not to mind colorful language, JoJo...doesn't. Except when the invisible hologram explains that she's part of a time experiment gone awry, she combines it with a spoonerism:

JoJo: "What the talk are you fucking about?"

In Hogyoku Ex Machina, after being told by Ichigo (who has traveled from the future) that his plan to take down the Big Bad by acting as a mole and assassinating him will ultimately not work, Gin's eyes are wide open and he says, “Well....shit.”
In the fifth chapter of this Death Note fanfic, Mello finds out that Matt scored a lot higher than he'd assumed. It turnes out the two of them made a Side Bet, with Mello assuming that he'd win. Well it turns out things didn't go that way...

Matt: I'm getting a codename.
Mello: You're kidding me
Matt: Nope
Mello: So that means...
Matt: That means for Halloween, you're going as Peach.
Mello: Shit.

In Turnabout Storm, from Phoenix Wright's part right after he discovers that Trixie, the prosecutor of the case, knew about Rainbow Dash's possible motive.

Phoenix Wright: Dammit! She knows!
Twilight Sparkle: Phoenix! Language!

Mass Effect Interregnum:

Garrus: I don't say this often, so understand that I really, really mean it: go fuck yourself.

Naruto Veangance Revelaitons is typically far too laden with profanity to qualify, but one happens in an ending added by the author's stepbrother, in which Edfred returns everyone to normal and Sakura kills Ronan

Sakura: FUCK. YOU.

The first sentence of Ccapter seven (or six, not counting the prologue) of KOTOR The Prodigy Of Revan Book One ends with Carth noting Jack's mood isn't the most ...civil.
- Considering the whole Zalbaar-being-captured-by-Gamorean-slavers circumstance, it's understandable
In Reconciliation, this happens at two different points in a single chapter, as Lilly is undergoing an emotional breakdown after her husband Hisao's funeral.

Akira: "Help me to understand here, Lilly, because right now I’m getting really fucking scared. I’ve never seen you like-”
Lilly: “That’s bullshit. My husband is dead. He’s dead, Hanako, and he’s not coming back.”

In Ace Combat: The Equestrian War, Firefly does two F-Strikes, both being aimed at Rainbow Dash. There are different reasons for it, however.
- In chapter 12, while Rainbow is suffering from a Heroic BSOD:

Firefly: You know what? TO HAY WITH YOU!

- In chapter 16, when Rainbow tries to calm her down:

Firefly: I CAN’T CALM THE BUCK DOWN!

Night of the Seance has this moment upon finding out what had happened to Frida.

"Forgive me for saying this, but holy shit!" Andy said.

In The Dark A.J. McLean gets called a twat by Melanie Chisholm for Flipping the Bird at her, checking her injuries. Chisholm still knew what that means.
- Though profanity is casually used, Emma Bunton, who normally a Kid Appeal Character, dropped many of these during her The Reason You Suck Speech in front of Victoria Adams (Beckham) and Geri Halliwell after she calls them out for rushing through the treatment Chisholm is going under.
Combined with This Is for Emphasis, Bitch in the Mega Crossover fic The Secret Return of Alex Mack when Alex -- who normally can't bring herself to call anyone, no matter how evil, anything harsher than "jerkhead" -- faces her nemesis Danielle Atron at the climax of the story and says, "Let's dance... bitch."