Brick Joke/Fan Works

Heir to the Empire by Ozzallos has a beautifully-executed example which might slip right past an inattentive reader. Early in the story, after Ranma is transformed (slightly) by his possession of the Ginzuisho, Kasumi admires his girl form's new silver hair and asks if he can give her silver hair, too. Ranma's response is distracted, bemused words to the effect of "sure, if I can figure out how I got it." In the last chapter, a hundred thousand words or so later, a teenaged Chibi-Usa visits from the future, stares at Kasumi, and asks "Say, when did Kas-chan dye her hair brown?"
In the prologue of Kyon: Big Damn Hero, Kyon compares being hit by Asakura Ryouko to being hit by a speeding minivan. Guess what happened over twenty-five chapters later.
Early in the Ranma ½ Crossover fic Desperately Seeking Ranma, Soun calls Nabiki and asks where he can find certain photo albums. Nabiki tells him where in the attic they are and (in an apparent joke) warns him about the attic spiders "that'll take your arm off!" Several chapters later, Akane goes into the attic to return the albums to their place, comes back down, and phones an exterminator, saying "Bring poison. No, that would take too long. Bring fire."
- And a few hundred thousand words later, it comes back again when Ryoga finds himself in the Tendo attic...