Revealing 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 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.
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.
Continuity mistake: As Jason breaks into Tommy's room, a purple monster mask on Tommy's wall starts to spin, then stops a few shots later, then starts spinning again in the next shot.
Revealing mistake: When Trish and her mom are jogging, a bush in front of them moves before they run by it, caused by the cameraman brushing it as he walked backwards.
Continuity mistake: When Sara goes into the second floor bathroom to dry her hair, there is a bench in the hall. Later, when Jason chases Trish upstairs, the bench has moved across the hall.
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.
Continuity mistake: Trish swings the machete overhanded, but in the next shot she knocks Jason's mask off underhanded.