Continuity mistake: In the opening scene, the head of Mrs Voorhees that Alice finds in the refrigerator is a completely different head that the one of her on Jason's shrine to his mother at the end of the movie. They look nothing alike.
Continuity mistake: When Jason surprises Vicky in the bedroom death scene. Jason pops out of the bed and his bag that he wears on his head is tied up. Then when they cut to Jason lowering his butcher knife, his head bag that he wears is un-tied. (01:02:20)
Continuity mistake: When Jason slits Scott's throat, he slits straight across, yet the wound is diagonal. (00:48:25)