Revealing mistake: When Brenda's body is thrown through the window, the movement of her head when hitting the floor is not "limp". She positions her head for the landing, then deliberately tilts it to the side as if dead. Also, on the second cut from Alice to Brenda we can see Brenda's head moving ever so slightly. (01:14:17)
Revealing mistake: After the girl escapes the moving truck, and stumbles through the woods, the killer catches up to her, and slashes her neck. It's obvious she's wearing a prosthetic, as it does not match her skin tone.
Visible crew/equipment: At the diner, when Sandy walks to the other end of the counter and talks to Steve, the reflection of the boom mic is visible at the top of the window behind Steve. (00:48:10)
Continuity mistake: When Ned fakes drowning he is a decent distance from the dock. When he is rescued he is much closer.
Character mistake: When Annie is in the red truck, she says that she'll be cooking for ten staff. When Alice goes to the beach, she tells Bill that everyone has arrived except for Annie. Even if she had arrived, there would only be eight of them: Christie, Annie, Ned, Brenda, Jack, Marcy, Bill, and Alice. Two shy of what she told the truck driver.
Factual error: There would be no slide latch on the inside of the pantry that Alice hides in before hitting Mrs. Voorhees with the frying pan. It would be on the outside.
Answer: 1. She was most likely hanged. 2. Mrs. Voorhees wasn't possessed by Jason. She simply went insane when Jason drowned and wanted revenge. 3. He's not burnt, only disfigured.