Continuity: The launch in which the Bowden family travels to the houseboat has a full windshield. Later, when Sam leaves by himself, only the windshield's frame is present; the glass is gone.
Continuity: When Sam is at the boat dock he goes into the shop and gets some thinner. As he is leaving the shop he has the thinner in his left hand. Then when he confronts Caty the thinner is gone and then he takes his daughter down the stairs the thinner is never seen again.
Revealing mistakes: For some strange reason, just after Sam shoots Cady, the shot of Cady from Sam's POV shows the branches of a bush superimposed on the bottom and left side of the frame. The tipoff is that when the camera moves the branches of the "bush" do not - they move along with the camera, as if they're glued onto the frame (which they are).