Livejournal Kink Memes are a spawning ground for these. The Ace Attorney one in particular (located here and currently at nineteen parts, as you can see) has generated piles of fanon, including a fic (and sequels) with Phoenix being the prince of Lithuania (AND moonlights as a porn star with the stage name "Earl Zach"), at least one original character (the most well known/popular being Hannah, Edgeworth's much put-upon secretary), an in-universe online rpf archive called "Lawyers in Love" and more besides.
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Hitler had become a Super Saiyan. Explanation The most well-known of all Crack Fics, famous for its endless absurdity, in which Goku travels back in time, falls in love with Anne Frank, and fights Adolf Hitler, who becomes a Super Saiyan, prompting this line. And he pulls it off in The Legend of Koizumi, too!
Pepper jack cheese. Explanation From a fanfic where the author had Hermione Grainger eat pepper jack cheese because (as stated in an author's note) it was the author's favorite type, the phrase refers to any case where characters conveniently like what the author likes.
Hrm, Half Life: Full Life Consequences hasn't been mentioned. Why not? BECAUSE YOU ARE HEADCRAB ZOMBIE. Explanation Another legendarily terrible fanfiction that spawned endless memes, this one actually spawned more from the animated version instead of the original text.
ORIGINALCHARACTER/IDEA!DONOTSTEAL! Explanation A standard disclaimer at the beginning of a vast majority of fanfics, especially those lacking in any originality, and a very good indicator of Unwarranted Self-Importance.
"Surprise!" Riker said as he boned young Wesley up the ass. Explanation From "Star Trek: The Next Fornication", it is basically a one-sentence summary of nearly every Star Trek: TNG citrus fic ever made.
Rude Person, the traditional Kirk/Spock Slash Fic plot parodied in different dialects.
Speaking of Kirk/Spock, thanks to the legendary and infamous "Closer" video, "Closer" is now the unofficial theme song for "Amok Time", has been played at several non-slash Trek conventions, and has spawned about a hundred copy-cat videos for different fandoms.
Hermione had changed over the summer. Explanation For years, Harry Potter fanfiction authors would find it necessary to make Hermione "more attractive", with almost an exact duplicate of these lines used somewhere along the way. So infamous that even Saturday Night Live parodied it.
She had curves in all the right places.
Her hair was no longer bushy, but curled perfectly and gleamed in the sun.
IT COULD GET ON DISNEYLAND ATTRACTIONS BY ITSELF.Explanation One person discovered a porn fic in which one of the characters wielded a 42 inch penis, and decided to share this discovery with the internet. Cue endless jokes such as this. And this: While I applaud your attempt at incorporating the famed number "42" from Hitchhiker's Guide to the Galaxy, and am quite amused at the Freudian association between Draco's penis and the reason for Life, The Universe, And Everything... a smutfic really isn't the appropriate venue for such subtleties.
THAT'SBULLSHIT!(eeps, covers mouth)Explanation Hinata learns that Naruto didn't get the It Gets Easier speech from the Ninja Academy's psychological advisor, but was cruelly dismissed as a monster who'd enjoy killing (this happened after Naruto's first kill during a mission). Hinata is outraged...and this was her response.
The Poultry Coup aka The Glorious Chikin Revolution!Explanation Do you remember those random chickens running around in Labyrinth? They are plotting to take over the Labyrinth and then (probably) the WORLD! It's the implied source of Jareth's crippling fear of Black Chickens.