Cassandra Clare's tale of shadowhunters and downworlders actually began life as Harry Potter fanfiction — Draco-Ginny fanfiction, specifically — before morphing and climbing its way up the bestseller list. So it cast actors who either had appeared in the Potter franchise (Jamie Campbell Bower) or certainly looked like they could have (Lily Collins), and hoped for the best. But the film cost almost $60 million to make and, with its opening weekend garnering a little over $9 million, was an instant flop. You've probably forgotten about it already. It didn't really have much crossover appeal for adults; Salon's reviewer spoke for many of us over the age of 15 when he said, "It's safe to say there was a lot about this movie I didn't quite get." For some reason, a sequel is in the works. There are still a lot of suckers in film financing.
Lesson learned: Thinly-veiled, derivative fanfiction might be popular with readers, but people aren't necessarily going to pay $13 to see it enacted on the big screen when such a wealth of it can be obtained for free on Livejournal.