Continuity mistake: When Jason crashes through the cabin wall and slashes the teen, a crate on Jason's right foot alternates from being on / off between shots.
Continuity mistake: When the three girls are sitting outside the school, Gibb puts her cigarette to her lips twice.
Continuity mistake: The end of Jason's machete changes throughout the whole movie. Sometimes it has a rounded end, sometimes it has a pointed end and so on.
Continuity mistake: At the end, Jason tears off Freddy's arm but when they go flying into the lake you can see Freddy has both arms intact.
Continuity mistake: Linderman accidentally shoots a Glock 17 in the van, grazing Will's right ear who grabs it, but during the remainder of the film there isn't a graze on his ear at all.
Chosen answer: Jason survived due to his unique healing abilities, at the end of the original "Friday the 13th" he is shown coming out of the lake. He is deformed with some mental handicap.
MasterOfAll