Paranormal Activity and District 9? 
I can't see how either of these fit the definition of a sleeper hit. Both had huge viral campaigns prior to their release. D9 had Peter Jackson's name slathered all over it from day 1, and that alone drew interest from many people. PA was touted as one of the scariest movies in years, with unique trailers which showed audience reactions rather than the actual film. It was a striking advertising campaign, and one which created a huge amount of interest prior to release. Just because it made enormous returns due to its tiny budget doesn't make it a sleeper hit, it makes it successful. I'm removing both of these from the list unless someone can explain their presence there. 67.133.229.226