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.
Plot hole: Jason drags Vicky's body down the stairs. A minute later, Ginny and Paul come in the house and see nothing. Jason even has time to hide Sandra and Jeff's bodies by the time Ginny and Paul go upstairs. Not possible in a two minute time frame.
Plot hole: When Jason is in Ginny's cabin, she's under the bed seeing where he's at. So how did Jason manage to get up on a chair and wait for her to come out without her seeing him do this? She had to see him do that, but when she pokes her head out, she's surprised to see him there.
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.
Plot hole: When the counselors are dividing up who is going into town to the bar and who is staying, Paul tells them only a limited number can go because there are only two cars. They leave in a little bug-looking car and a pick-up truck. Then, when the girl is freshening up for the guy in the wheelchair, she goes outside to what is presumably her car, searches the front seat, and finds the hair brush she is looking for. I guess technically, there are only 2 cars and 1 "truck" but the truck is the other vehicle that goes into town, so presumably he meant 2 "vehicles" which would be incorrect since a 3rd was obviously left behind.