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)
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)
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)
Revealing mistake: Near the end when Jason throws the hammer at the girl, it sticks to the damaged part of the wall after it hits. But it's just against the wood with the flat top, there's nothing to hold the hammer there and it's only hit the wood on the other side of the wall. They probably used a magnet to attract and keep it there for safety reasons. (01:15:00)
Revealing mistake: When Jason rips Jimbo from the door, it shows a close up of his hand. Looks and tears like rubber. (01:17:15)
Revealing mistake: Just before Trish jumps out of the window, as Jason runs forward to get her, you can see that he has normal, human eyes, instead of his disfigured, strange eyes, seen at the end of the movie. (01:18:00)
Revealing mistake: When Trish jumps out the window and falls to the ground you can see her using a body suit covering her from her hips and down her legs. (01:18:15)
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. (01:18:15)
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. (01:19:20)
Revealing mistake: Trish jumps out the window and lands face up on the ground. You can plainly see it's Kimberly Beck's stunt double. She looks nothing like her. (01:21:35)
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." (01:23:15)
Revealing mistake: After being hit with the machete, as Jason falls and lands on the ground (with the machete holding his skewered head in the air), it is painfully obvious from the skin texture and unnatural facial movements that his head is a replica prop. (01:25:00)
Answer: You can see from the inside of the house Gordon jumping through the window by himself. It shows him jumping on the table then through the window.
lartaker1975