(at around 37 mins) When Billy is waiting for Lana outside the diner, his car stereo is on and playing loudly. Yet, when Lana comes out to the car, the radio is then off. It is very unlikely that Roy turned it off.
(at around 34 mins) When Billy pulls up outside the diner to pick up Lana, if you look into the window at the far right of the screen you can see a crew member hide into the corner of the diner so he won't be seen.
The goof items below may give away important plot points.
(at around 46 mins) When Jason is killing Eddie by squeezing his eye sockets with the leather strap around the tree, Jason turns the rope counterclockwise first but when it shows it snap from pressure he is turning it clockwise.
(at around 1h 7 mins) When Violet is dancing in her room, Jason starts to walk up to her and we see the knife has dried blood on it. Then he picks her up, stabs her takes the knife out and now the knife is clean.
(at around 1h 25 mins) In the final scene, when Sheriff Cal Tucker is explaining to Pam the motive for Roy the paramedic snapping, he shows her a series of newspaper clippings about Jason Voorhees and his past. In one of the clippings is a picture of Jason standing and looking at the camera. How would anyone have been able to snap a picture of Jason to be printed in a newspaper?
(at around 1h 23 mins) In the final close-up of Jason when he is impaled on the spikes dead in the yard, you can just see his right eye flicker for a split second, probably due to unexpected water droplets from the rain.