Continuity mistake: At the start in the van, the girl you see smoking dope hands over her cigarette to the other kid. She uses her index finger and her thumb with the cigarette facing downwards, but in the next shot she has got it between her index finger and middle finger and it is facing upwards. (00:18:35)
Visible crew/equipment: When Loco is watching the girls swing from a rope in The Barn Loft, he bends down to full with the gas can and when he looks back up she's gone. If you look closely by The Barn Loft door to bottom right there is someone moving with a blue jean leg. Jason is in dark khakis. So this is obviously crew.
Revealing mistake: Inside the barn, the webbing that the deer head is covered in is fake, as it 'pulls' like elastic when Shelly moves his along the skull. Real webbing would adhere to his bare skin.
Continuity mistake: When Shelly and Vera are walking out of the store, Vera stuffs Shelly's wallet into her beige pants. Later, while sitting on the dock, she pulls it out of her pink pants. She doesn't have a change of clothes with her. (00:31:05 - 00:56:40)
Continuity mistake: Jason has dark Khakis throughout the film, until the very end. When the cops are at the house, his body is in the barn wearing blue jeans. In part 4 he is still in blue jeans, but then changes into dark Khakis after leaving the hospital.