Revealing mistake: You can see the stick making the finger move when it's in the box. (00:38:20)
Other mistake: When Jody finds the dwarf in the car and recognizes it to be his deceased friend Tommy, he has a quick flashback to Tommy in the coffin and the exact moment Tommy was murdered. But the second flashback makes no sense, Jody never saw Tommy being killed and was told by Reggie that he committed suicide.
Continuity mistake: At the beginning of the movie, when the girl is having sex with a guy, her legs are parallel to his, then it cuts to her sitting up on him. It's not physically possible for legs be flat on the ground whilst sitting up like that.
Continuity mistake: When the door slams on The Tall Man's fingers, you can clearly tell that no fingers are really sticking out of the door when the door actually shuts. (00:36:10)