Other mistake: When Carter is basically trying to kill himself with the remaining "survivors" in his car, Billy makes a comment saying that when he saw Alex leaving Mrs. Lewton's place, Alex's shoe prints were in her blood and his prints were on the knife. All good and fine but there's just one thing that doesn't make much sense. Her house blew up right after he ran out, so could anyone still pull off any prints from the knife that killed her, and see footprints by her body? Wouldn't that evidence be gone in the explosion?
Continuity mistake: When you first see Carter's car you can see that is says "SS" on the front. Then when he almost hits Billy at the school it doesn't say it. Then when he is on the train tracks it says "SS" again.
Revealing mistake: When Alex is running through the woods and is sliding down the hill, just before he stops himself from being impaled by some branches, you can hear someway say "Look out." It can't be the agents following as they're too far away.