I learned on TV Tropes... that apparently there are people out there who write fanfic for fandoms whose canons they have not seen/read/etc. at all. They literally write fanfic based on... other people's fanfic. And what they've been able to learn/assume about the canon from that fanfic. Though it's arguable, if say, the movie is really awful but existing fanfics from the books are really good, if maybe that would be preferable ;)

It's funny, since Phantom, you know, is one of my fandoms... and I got into it because of the movie of the musical, and I was SO obsessed with that movie and thought it was the best thing since sliced bread. Coming into the fandom, probably fortunately, I ran into all of these bitter old fans from before the movie, and I was exposed to stuff from the musical on stage, old movies, the book, etc.

Now I've disowned that movie.