Revealing mistake: When Jason is whacking the party goers, you can see the padding Jason slices under their shirts.
Revealing mistake: When Jason cuts off Freddy's arm you can see the actor has his arm tucked behind him under his sweater.
Revealing mistake: When Jason swings his machete at Blake on the porch, as he swings you can see the actor stop swinging so it doesn't even make contact with Blake 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