Clay can be seen briefly in a few scenes with a fixed blade knife in a sheath on his belt. Not only does he never think to use it to defend himself against Jason, he looks frantically for other weapons in the barn right before Jason crashes through the window behind him. Did he forget that he had the knife?
(at around 38 mins) After the verbal altercation with Trent at the lake house, Jenna leaves with Clay Miller, who is carrying a satchel. As they approach Miller's motorcycle, notice how the position of the satchel changes from being strapped around Miller's left shoulder, to being carried in his right hand, and, finally, back to his left shoulder.
(at around 44 mins) When Chelsea is wake boarding, when she first gets up, she is clearly putting her left foot forward () however the first shot immediately after this, her right foot is forward (goofy), yet every other shot she is wake boarding with her left foot forward (regular).
(at around 26 mins) Clay puts one of his missing persons poster on the back of the Escalade and there's a close up shot of the poster as the vehicle drives away from the outpost store. The ensuing shot is of the vehicle entering the city limits of Crystal Lake, passing by the sign, however there's no poster on the back of the vehicle.
(at around 1h 5 mins) When Jason is walking up to the house and sees Trent and Bree having sex through the bedroom window, you can clearly see the ceiling fan above them is on and spinning. In the next scene which is a close up of Bree from the waist up, the ceiling fan is not moving at all.
(at around 1h 5 mins) When Trent and Bree are having sex in the bedroom. Bree is seen sitting up with her hands on top of her head holding her hair. When it cuts to showing Trent's upper body half. You can see Bree's hands by his neck and shoulders. When it cuts back to the same side view as before with Bree sitting on top of Trent as he is laying down. Her hands are all of a sudden back at the top of her head holding her hair. As quick as the shots are shown there is clearly no time for here to get her hands back up that fast.
At around 38 minutes, when Clay is leaving the house, he has his backpack on his left shoulder, when the camera angle turns, he has the backpack in his right hand.
When Clay get on his bike, he has his backpack in his left hand and put it on the bike, when the camera changes again, he sits back again on his bike but the backpack is again on his left shoulder.
(at around 11 mins) When the youngman finds the weed plants in the woods, he smells the leaf of a plant. Actually, it's the flower of the plant (the smokeable part) that gives the famous smell, not the leaves.
(at around 9 mins) Wade explains that GPS unit works by sending a signal to the satellites which in turn respond; in reality, GPS units only receive signals from various satellites and never transmit any signals.
The goof items below may give away important plot points.
(at around 46 mins) Chelsea gets hit by the boat after falling off the wake board, and when she emerges from the water, we see her holding her head as there is blood from her head. When she hides under the boat pier whilst Jason is on top of it looking for her, the blood is gone and there is no visible injury.
(at around 1h 12 mins) Whitney has escaped Jason's dungeon and runs to the window where she sees Trent and Bree having sex. As she's about to pound on the window for help, a close-up shows Jason put his hand over her mouth and pull her away. Then, the angle changes to a far shot and Jason puts his hand over her mouth and pulls her away again.
(at around 1h 11 mins) When Lawrence gets killed with the ax there is no mistake. Jason throws a double sided ax into Larry's back. Larry falls forward onto logs screaming. Jason then comes behind Larry turning him over and stomping on his body once. this causes one side of the ax to come through his chest. The other side of the ax is still in his back and was never removed. That was the last we see of Larry.
(at around 1h 16 mins) When Jason throws Bree out of the house onto the police car, the windshield breaks like house glass. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two curved sheets of glass with a plastic layer laminated between them for safety, and don't shatter so much as crumble.
(at around 1h 30 mins) In final scene Jason's mask strap is ripped hanging off of the mask. Then it is reattached when he comes out of water. But him returning may have been a dream of the final characters. In that case then his mask could be in any condition.
(at around 1h 5 mins) When Chewie gets stabbed right under his chin, the position of the screwdriver changes from dead center to far right (away from the camera) so much that you clearly see it's too far over to actually be in him, more like it's on the outside of his neck.