The Blair Witch Project

The creators of Blair Witch famously duped early film-festival-goers into thinking that the film was genuinely amateur footage discovered after the three filmmakers went missing - something made entirely plausible by excellent improvisational acting and the fact that nothing actually happens. After the film's sleeper hit success in the states, the creators were in turn duped into putting their names on the lamentable, commercially driven sequel, in which they had little involvement.