Revealing mistake: Just before Walter throws Lisa through the window, you can tell that Lisa is a stuntman wearing a wig.
Continuity mistake: When Walter is being rescued from the room he was trapped in, through the hole in the wall, we see the outside of the hole and Walter's body is outside except for his legs, and we see the hands trying to pull him back. In the next shot inside the room, Walter is again inside except for his head and arms. It repeats several times. (01:28:30)
Suggested correction: This is incorrect. Walter and Lisa are wearing similar clothes, making it look like he is in both rooms at once. Instead, as he is almost completely in the other room, Lisa is close behind him.
Continuity mistake: Susan and Lisa are standing perfectly still for a long time awaiting the transformation of the possessed girl, yet Lisa's hair goes from being behind her back to over her front then back again when the possessed girl "wakes" up.
Factual error: In the movie's opening scene, it's night and we see the full moon. There's an elderly priest lying in bed. The next morning we learn the priest has suffered a stroke and been taken to the hospital. That day, Prof. Birack pauses before entering a campus building to look toward the sun. We see a thin crescent moon near the sun, as if there will be an eclipse in the next few days. That night, Brian is waiting outside a campus building for Catherine to come out. He looks up at the full moon. In following days, other characters see the daytime crescent moon, still approaching the sun. It's impossible to see the full moon at night AND a crescent moon during the day in that brief a time span. It would be more like 12-14 days for the lunar phase cycle to progress from one to the other (much less back again).
Continuity mistake: The hand of the possessed girl calling her father (the Devil) through the large mirror varies between being a normal hand with minimal makeup to closeup shots with it heavily laden with makeup and thick fingers.
