I think it's safe to say that J.K. was planning this reveal for a long time. She didn't just pluck it out of someone's fanfiction on a whim. But it's still true that a few Potterheads predicted that Harry was one of Voldy's horcruxes long before book seven came out, and all the other fans thought they were nuts. Vindication is oh so sweet (but Rowling did shut down that whole theory that the Dursley's hated Harry because he was a horcrux—turns out, they were just huge jerks).

Fans of hard sci-fi are none too easy to please. When Larry Niven's Ringworld came out in 1970, it was an instant science fiction classic. Readers loved it so much that they took it upon themselves to start writing letters to Niven... about all the things he got wrong. According to physics, fans insisted, the titular Ringworld was unstable, and would soon drift into its own sun. Rather than get defensive, Niven sat down and wrote a sequel in which the Ringworld was indeed in danger of crashing into the sun, which is pretty much the best thing anyone has ever done with a fan critique.

If your fan theory gets rejected by the creators, you can always pull a Geoff Johns. In 1996, as a young comic book fan, Johns wrote in to DC Comics to suggest that maybe the young hero Superboy was a half-clone of Superman, and a half-clone of Superman's nemesis, Lex Luthor. Johns made it into the "letters" column, but the writers didn't take him up on his idea. In 2003, however, a grown up Geoff Johns got to make his wild theory an official plot point as the author of the new Teen Titans series, and it's been Superboy's official backstory ever since.

Once you fall down the well of A Song of Ice and Fire fan theories, there's no coming back. It's a little unclear what can be considered "official" or not, since the HBO show has zoomed past the books and started confirming and ignoring a whole slew of fan theories. But we can cling onto one reality in this uncertain time: the Three-Eyed Crow is officially Brynden "Bloodraven" Rivers. Fans spent years theorizing who this creepy, time-traveling tree-bird-man was, and in book five he admits to Bran Stark that he used to be a lord called Brynden, making him a former Hand of the King and a bastard Targaryen (just like every other freaking character in those books). Pat yourselves on the back, everyone who was on team Bloodraven.

OK, so this isn't exactly a theory that "came true," so much as a theory that made the author go, "Sure, OK!" In Philip Pullman's His Dark Materials series, human beings have an external soul or "daemon" in the form of an adorable animal that hangs out with them all the time. Most people's daemon's are the opposite gender from themselves, but a few rare individuals have daemons of the same gender. Many fans suggested that people have daemons of the gender that they themselves are romantically attracted to, and Pullman accepted this theory in an interview (but what about the daemon's of bisexual people, Philip? What about pansexual people? The new trilogy had better answer all of these questions).

Sir Arthur Conan Doyle famously got sick of writing Sherlock Holmes stories, and decided to kill off his beloved protagonist by having him fall off a waterfall. But even in the 1890's, a little thing like death by waterfall didn't stop fans from theorizing. So many readers demanded that Holmes come back to life, and concocted stranger and stranger ways that he could have survived, that Doyle finally relented and gave into the fan theories: Holmes did indeed fake his own death, climb down a cliff, and spend a few years kicking around Tibet before rolling up in London to bother Watson again.