Other mistake: After Michael kills Laurie at the beginning, he hands the knife to the inmate with the clown mask. The next kill is the cameraman for which he uses the tripod ends. A few scenes later they show Michael standing holding a knife with fresh blood on it. He hasn't killed anybody with this knife yet so where did the blood come from?
Other mistake: This is a timing mistake. In the beginning of the movie, when Michael kills the first security guard, he stuffs his head in the washer. The second security guard was in the room within 30 seconds. There is no way that Michael had time to cut this guys head off, stuff it in the washing machine, turn it on, and hide before the second guard got there.