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.
Visible crew/equipment: When Ginny and Paul walk down the stairs after they found the bed drenched in blood, Paul walks by the camera and you can see the camera's shadow on his chest. You can tell it's the camera's shadow, because when it cuts Ginny walks the same way and no shadow falls on her chest.