Plot hole: Every ghost that we see in the house looks the way they did when they died, for example the girl from the film's intro is seen without skin on her face, which happened to her before she died. However, in the final scene when we see Simon's ghost joining them, he looks completely normal, when he should have had a throat slit wound and the carved markings on his skin. (01:31:20)
Continuity mistake: When the main characters visit Montreal and with their friends they go out to eat at a diner and he creates a "home" out of 4 toothpicks, 3 sugar cubes and a coaster. As the camera goes back and forth in the conversation, watch the "home" on his plate of pancakes. It falls and appears to be scraped and turned in a different direction on his plate by the end of the conversation.