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. (00:07:00)
Continuity mistake: Nurse Morgan's name tag is on the opposite side to the pocket with the pen in it, on her white nurse dress. Yet when Jason stabs Nurse Morgan, her name tag is right next to the pocket. (00:12:00)
Continuity mistake: Right before Jason slits Axel's throat, we see him grabbing Jason by his elbow. When it cuts from a shot above we see him holding Jason by his wrist. (00:13:25)
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. (00:14:10)
Factual error: When Tommy is playing Zaxxon on his computer, he is about to go into a barrier and die. Firstly the audio of the ship crashing happens before it hits, secondly, in the next shot the ship has reappeared. (00:14:30)
Continuity mistake: When the six teens are driving along in the car singing, between a wide shot and closeup of the car, the road and scenery behind them changes. (00:15:35)
Continuity mistake: When Trish's car stops, the licence plate goes from dirty to clean. (00:28:10)
Continuity mistake: When Sam asks Sara to come into the water, Sara says no. Then Sam says "Then I'll go under water and stay there until you do", while placing her hands on the edge of the pier. When it cuts to a close-up of her she is resting her arms on the pier instead. (00:29:20)
Continuity mistake: When two of the teens are dancing to the rock music, one boy walks over, changes the record and then places a vinyl record on top of a box. In the next closeup the vinyl record he placed down has gone. (00:34:35)
Continuity mistake: When Jimmy and one of the twin girls go upstairs, Ted is left behind with the other twin. He tries to give her a charming look while both his arms arm down. When it cuts from behind him he is suddenly holding his left arm up. (00:43:45)
Visible crew/equipment: When Paul is stabbed by Jason's harpoon and lifted up in to the air, the wires attached to him are visible on either side. (00:45:40)
Continuity mistake: When Tommy is pretending to sleep when his mom comes into the room, he is on his stomach when she pulls the blanket over him. In the remaining shots starting with when his mom is closing the blinds, Tommy is suddenly laying on his side. (00:51:15)
Continuity mistake: When Jimbo gets his hand impaled on the counter with a corkscrew by Jason, the counter is clear, but a few scenes later as Jason leaves the kitchen, there's a pot holder laying on the counter. (00:58:25 - 01:01:20)
Continuity mistake: Jason slashes Jimbo in the face with a cleaver. It then cuts to another scene happening elsewhere. Then when it cuts back to the scene with Jimbo, Jason now has a large bloody knife instead. (00:58:30 - 01:01:20)
Revealing mistake: When Tina is thrown out of the window by Jason, you can see she is wearing soft body suiting on her legs as she hits the car. (00:59:25)
Revealing mistake: When Tina is thrown out the window you can see the roof of the car is partially damaged and very flexible when she hits it. (00:59:25)
Continuity mistake: When Jason kills Doug in the shower, there are two red lines on his hockey mask, under the eyes. However later, when Jason bursts through the window and grabs Tommy, the lines are gone. (01:05:10 - 01:14:45)
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. (01:06:30)
Continuity mistake: When Gordon jumps out of the window, shards of glass breaks off, flying all around him as he jumps. In the next shot all the shards have just disappeared. (01:10:05)
Revealing mistake: Trish tells Tommy to go get a hammer and some nails. When Trish then shouts "Tommy, hurry" you can see the shelf to her right get bumped by somebody in the crew, who was obviously standing nearby. (01:13:50)
Answer: Jason killed her. There was a scene shot but never shown where Trish discovers her mom's body.
SAZOO1975