Revealing mistake: When Loomis shoots the marshal's window out, when he makes a u-turn you can see the tire marks on the road and dirt from previous takes.
Revealing mistake: At the end of the film when Ginny and Paul are fighting Jason, his machete hits the shrine where his mother's head is displayed and where Terri's body is strewn. When the machete hits, Terri flinches, even though she is dead. (01:16:00)
Revealing mistake: When Wally is firing the laser at random from the small spaceship, we see Fidget running down some stone stairs, and there are violent laser impacts directly behind him that could only have hit where they did if they had passed right though his body. (01:41:55)
Revealing mistake: When the two station reporters are investigating Eddie's room, as the female bends down to the table, she is scared by a dog. In one shot you can see a black wire on the dog. (00:14:55)
Revealing mistake: Burke kills Mary Roberts. She blinks when he turns her head to check out her face. (00:47:10)
Revealing mistake: The Prowler's just killed Lisa in the pool, yet he's walking across a lawn completely dry. Clearly this is footage reused from when he was outside the dorm earlier.
Revealing mistake: During the flashback to Ginny's operation, you can tell it's a prop head due to the color of the part where the operation's taking place.