Other mistake: When Jeff and Sandra sneak off to look at Camp Blood, they come across a dead animal. While they're bent down looking at it and have their backs to the camera, the cop for some reason sneaks up behind them and puts his hand on Jeff's back. The camera angle changes and you see the police car parked just off to the left with its red light flashing. There's no way that Jeff and Sandra would not have seen and heard the police car when they came up on the dead animal - it's almost right in front of them.
Other mistake: Vicki enters the darkened cabin and flips a switch on a nearby lamp, to discover the power is out. But as she grumbles and crosses to another table lamp that is unplugged, she plugs it in, and suddenly the whole cabin lights up.