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?
Plot hole: When the first security guard, who stayed back to get something to eat, hears something in the laundry room, he walks over to the washing machine where he finds the head of the other security guard inside. He turns around screaming and trips over the security guard's body on the ground with no head. Wouldn't he notice the dead body on the ground when he first entered the room?
Question: It was revealed that the person who Laurie thought she killed wasn't Michael but somebody who Michael had subdued and placed his clothes and mask on. Why would Laurie be sent to an institution for this? She wasn't aware of the fact that she killed the wrong person until the mask was removed, and since the staff seem to know about her connection to Michael, wouldn't they consider letting her go since she was only trying to stop Michael's rampage?