Continuity mistake: When Trish has entered her house after escaping from Jason, she hears him behind her in the doorway, and swings at him with a machete, but hits the door frame. You can see there's a mark from a previous swing.
Revealing mistake: Near the end when Trish is running from Jason, Jason crushes a lamp on the floor and for an instant it becomes pitch black. A moment later, the room is lit again by a sudden occurrence of "moonlight."
Plot hole: When Jason first wakes up in the morgue, he kills two hospital employees inside the hospital (a semi-public place) and leaves them there. Later in the movie, Rob Dyer (the hitchhiker)in trying to convince Trish Jarvis that Jason Voorhees is still alive, he says that two hospital employees disappeared. Their bodies were not discovered in the middle of a hospital?
Visible crew/equipment: When Terri goes outside to go home, as she walks towards her bike, you can see someone upstairs walking by the window. Must be a crew member because Tina and Jimbo are in bed having sex and Ted, Sarah and Doug are watching the film downstairs. Paul and Samantha are dead. There is nobody left in the house.
Plot hole: When Jason is going next door to chase after Trish, he pulls Jimbo's body down from the doorway and begins walking towards Trish's cabin. He is seen walking by himself. However in the next scene, he throws Rob's body through the window. There was no time for him to go back and get the body.
Continuity mistake: In Part III, Jason's fingernails were neat and trimmed, but in Part IV (which occurs the very next day), they are long and black.
Revealing mistake: When Trish jumps out through the window, rolls off the deck, and lands on the ground below, she obviously lands on a mat or some other protective device covered to look like mud. It jiggles like jelly in a perfect square when she lands on it.