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 she gets the pizza from the guy at the door, it is in a white box. When she goes in and sets it on the counter, it is in a brown pizza box. Then later, when she is running away covered in blood, she grabs the knife on top of the pizza box, and it is white again.