Other mistake: In the beginning of the movie when Kimberly is shackled to the floor you can see that the padlock isn't locked. She could just take it off and be free.
Continuity mistake: After the dummy is in the water with the camera and the killer comes back to check on his "garden", we see the dummy from behind while he's swimming towards it. His hair barely moves. Then we see the dummy from the front and the hair is flowing freely in the water like everyone else's.
Factual error: When the husband swims to the other side of the lake to get some stuff in the truck, he lights his cigarette on one side and when he arrives on the other side, it's still lit. How could he swim trough the lake without getting his smoke wet?
Revealing mistake: The algae in the "garden" is fake. It is cloth.