My favorite crack theory is from the HP fandom: the "Knight2King" theory. It popped up after Order of the Phoenix, arguing that Dumbledore and Ron were the same person, and that Ronbledore had traveled back in time to help Harry. It also involved the idea (sometimes seen on its own) that the giant chess game in Sorcerer's Stone actually predicted all the major events/deaths of the rest of the series. (People got straight-up deconstructionist on analyzing every move in that chess game.)
WHAT THE FUCK
i mean
WHAT THE FUCK!!!!
oh my god i wish i could have witnessed that in real time. that sounds amazing.