When Jessica gets in the truck to go to the Voorhees house, there are large stripes of blood on her neck. They disappear when she gets out of the truck and reappear when she talks to Duke inside the house.
When the female FBI agent Marcus is running from Jason (leading him to the ambush) at the beginning, she was barefoot and wrapped in a towel. In one scene where she is running through the forest, she is wearing hiking boots and white socks. At the close-up of her running legs she is barefooted again, directly followed with a scene wearing the hiking boots with white socks again. When she leads Jason into the clearing, she stands up when the lights go on, and she is wearing some light-brown sneakers and in her right hand she has, what seems, a little gun. Then the FBI opens fire.
When Deborah and her boyfriend go into the tent, their shadows show her taking off her shirt and tossing it aside. In the next outside shot of the tent, her boyfriend is once again shown taking off her shirt.
Pamela Voorches is revealed to be a powerful necromancer who raised her dead son Jason from his grave despite the fact that she was killed in the first Friday The 13th movie before Jason had ever been re-discovered at the bottom of the lake.
In the first few Fridays, the cars have NJ plates and there are several signs stating that Crystal Lake is in New Jersey. The sign at the beginning of the movie that Officer Marcus drives by has Connecticut towns on it.
When Agent Marcus is running in the woods, after jumping over a low branch she is clearly wearing shoes, though she was barefoot when she ran out of the house and moments later she is again not wearing shoes.
In the tent, Deborah takes a condom packet from her boyfriend and tosses it to the side inside the tent. Later when Jason approaches the tent, the condom packet suddenly appears outside as he steps on it.
Steven is obviously not bound by real handcuffs in the police station, because when he jumps up and passes his hands under his legs, he spreads his hands a lot wider than real handcuffs would allow. If you look closely, there is just a piece of string binding his hands.
When the girl is impaled with a post in the tent, a large, bloody gash is already visible running up her body in the exact spot that Jason thrusts the post upwards; before he actually does it. This pre-cut on the dummy was done to give the pole a straight, clean line to move up in so it would look good in the movie.
The goof item below may give away important plot points.
When Jason is "reborn" as his spirit enters his dead sister's body, he returns in the exact same body he had before, complete with the same deformities and injuries, same torn clothing, and hockey mask.