Revealing mistake: When the killer surprises Sid after she's come in from outside and locked the door, he tries to stab her with his left hand, misses, and hits the door right next to the chain in closeup. In wide shot (very quick shot) when Sid legs it, look at the killer and you can see he's holding a very badly bent knife in his right hand, nowhere near the chain.
Revealing mistake: After Gale backs the van up, and Kenny's body falls on the windshield, they cut to a shot where Gale is driving away, and Kenny's body is on the roof of the van. When she starts driving you can clearly see Kenny lift his head into the air to look forward, even though he's dead.
Revealing mistake: When they shoot the killer at the end, the hole in his head is there before the gun goes off. You can see the hole in his head for about a frame before the flash of the gun goes off and his head drops. The body is also clearly a dummy. The scene then cuts and they show him being shot again. Once more, the wound is already there.