Revealing mistake: When little Melvin gets turned back into Toxie, Toxie charges towards the devil and picks him up, and starts swinging him up in the air. While he's doing that, you can tell the devil has been replaced by a very unconvincing dummy.
Revealing mistake: When the school bus is put on top of the cliff by the devil, you can tell they didn't get the bus to go up the hill. They actually dropped it of the cliff, then reversed the film and ran it backwards so it looked like it went up. You can tell by one of the small trees which is flat, but then it stands upright.