Other mistake: In the final scene of the movie, Megan starts a boat motor and uses the propeller to hit Jason in the face, who has been chained down underwater. But if Jason can only barely reach her foor from underwater, there's no way the boat propeller is underwater far enough to reach his face.
Continuity mistake: When Jason approaches the RV with Cort and Nikki inside, we can see that the vent on the roof (above the front of the RV) is wide open. It is still open when Cort starts driving away. In the next scene when the RV is driving down the road, and after it flips over, the vent is now closed.