Revealing mistake: When Ginny has run out to her VW Bug and is attempting to start it, Jason is supposed to be back in the house just on his way out to get her. But if you look at Ginny sitting in the car, in the lefthand corner of the windshield you can see a reflection of the top of Jason's hood. He was there all the time, crouched down beside the fender just waiting for his cue to pop up. (01:07:45)
Revealing mistake: Just after the scene where Mark is killed, there's a shot of Jason's feet as he opens a screen door and walks across a dusty hardwood floor. As the dolly pulls backward across the floor, you can see the camera's roller marks in the dust. (00:59:10)
Revealing mistake: When Jeff and Sandra are in the woods, they bend down to examine a mangled animal. You can see that the animal has human teeth, not animal teeth.