Revealing mistake: When Tommy goes to the Sheriff's station to warn everybody about Jason, Sheriff Garris ignores this warning. Tommy freaks out and grabs a shotgun but Sheriff Garris throws him to Deputy Rick, who drops the fast-food boxes he was holding. We can see and hear that these boxes are empty when they go flying.
Revealing mistake: During the Paintball game, Jason decapitates 3 players who are standing side by side all in one swing of his machete, unfortunately all 3 people are of completely different height to each other so there is no way he could have decapitated them all at once. Also, even a viewer with the poorest eye sight can see the boxes the 3 dummies are standing on when their heads fly off.