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. (00:42:15 - 00:57:15)
Continuity mistake: When Jason comes smashing through the window at the end, to attack Ginny who is getting up from the cot, the camera angles are inconsistent with the attack. First he is lounging at her from behind, to grab her. Then his distorted face is suddenly at her left shoulder before he's even touched her Then she's looking back at him and screaming before his hands even touch her and pull her out of the window. (01:24:20)
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.
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.
Revealing mistake: Notice when Jenny pees it's shown just pouring out. But she's lying flat and her jeans would obviously absorb most of it. But it just pours out like she's standing up with no jeans on. (01:14:30)