Continuity mistake: In the scene where the murderer is hacking down the front door with an axe, the door and its hinges change. In one shot the hinges are on the left (as we see it) and the door closes. The murderer then manages to hack through the door and the door swings open with hinges on both sides, opening like a swinging door from a western movie.
Continuity mistake: When Jody's dad (the sherrif) goes into the principal's office, you can see it's a small room with nothing to hide behind. He closes the door after him and seconds later the killer steps up behind him and knocks him unconcious. The killer couldn't have hidden in the room and the door opening would have been audible, so the killer appeared literally from thin air.