The events in the film supposedly take place during summer according to Boris' makeshift gravestone (Boris Tulinsky 3-5-1923 to 6-7-1980) but later in it, the two young women that encounter the fake herd of cows are dressed in winter clothing, are talking about a ski instructor, and have skis strapped to the roof of their car, implying that it takes place during winter.
In the picnic in the film's graveyard scene, Vincent elbows Ida sharply on her side under her right breast, but she immediately bends over and grabs the center of her stomach.
During the fight between the Sheriff and Vincent in the film, the Sheriff's shirt switches between being badly cut up to being in relatively good condition.
Bob sets his bag on the center front of the hood of his car, but when he goes back to pick it up, it has moved to the back of the driver's side fender.
Toward the end of the film, when the saw is turned on during the fight, the object on the "conveyor belt" would not index toward the saw. It is a "skatewheel" conveyor belt; it just has a series of rollers that only allow a person to easily push something down the line. There are no motors, belts, etc. to automatically drive or feed anything down the line toward the saw.
The dis-articulated torsos Vincent hangs upside down on hooks are anatomically incorrect. The crotch between the leg stumps lacks both genitalia and any wounds suggesting their removal. The way Vincent pushes the torsos down the line appears to be an attempt to disguise this.
Reflections of the crew/equipment can be seen in the Sheriff's glasses when he is first shown.
After Bruce runs everyone off to get a better spot so that he and Terri can see the drive-in theater screen, the camera shows the car only moves about 20 feet. They stop with the car pointed down into a dip in the land. They are not anywhere near the edge of a cliff or a drop-off. When he tells her "Look down there", all she would have seen is the other side of the dip.
Near the end of the film, when Terry is tied up on the conveyor belt and moving toward the saw at the other end, she struggled and screamed for help as she got closer to the saw, but she could easily have reached over to pull the looped ropes off of the three bolts on the side and freed herself, since the ropes were not tightly bound around her and her left arm was loose - not bound under the rope.