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)
Continuity mistake: The last thing Andy sees is he walks down the hall on his hands is Jason slicing straight downwards at him with his machete. But this would have bisected him lengthwise, not at the waist. When we see his body up in the rafters, he's been sliced into a top half and a bottom half.
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.
Other mistake: After Jason slashes Andy in the hallway we see the machete ripping down through Andy's abdomen as he is upside down in his handstand. Then his body falls to the floor and Jason starts dragging him, but there is no blood spatter anywhere on Andy's chest, face, or the floor.
Continuity mistake: When Loco goes to the barn, he has a cigarette in his mouth. When he goes into the barn, it has gotten shorter although it's not lit.
Other mistake: When Chris finds Shelly "pretending" to be dead, if you look as she opens the door, you will see the door lock on the knob is outside the door, not inside.
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.