Leonardo DiCaprio — Declined Anakin Skywalker

George Lucas personally met with DiCaprio to play Anakin Skywalker in the Star Wars prequels. DiCaprio said no because he "didn't feel ready." Hayden Christensen took the role — and became one of the most memed actors in cinema history ("I don't like sand"). DiCaprio instead did The Beach (a critical flop) and Gangs of New York. He's also reportedly turned down Spider-Man, Robin in Batman Forever, and Boogie Nights. DiCaprio's career turned out fine (six Oscar nominations, one win, $300M net worth), but an alternate-universe DiCaprio Anakin might have actually saved the prequels.