When Uncle Ned is electrocuted, he stands up. This would not be possible as the contraction of muscles would have prevented that.
When kids who got attacked inside Satan's Den come back with the police they are in the back seat of the police car and get out at the same time the cop exits from the driver's seat. This should not be possible since the rear passenger doors of police cars are designed to only be opened from the outside.
The set is supposed to be in the US in spite of having been shot in Italy. A truck in repair shop and Hardin's Porsche have fake Arkansas license plates. There is a round European speed limit sign in the repair shop scene and later a truck parked at roadside has an Italian license plate.