Revealing mistake: Charles is cutting a letter into his arm with a knife towards the beginning. However it's obvious the knife is fake and the blood is coming out of the top of the knife onto his skin like an ink pen on paper.
Revealing mistake: At the end, when the monster grabs the sheriff and throws him across the front yard, the sheriff lands on his back and his right arm falls into a beartrap. The trap snaps and the sheriff screams in pain, but only one jaw of the prop beartrap actually snaps into vertical position (the opposing jaw remains flat and doesn't move). A real beartrap snaps both jaws together vertically. So, the sheriff's hand doesn't get snapped at all.
Answer: Sleep Walk by Santo and Johnny.