Character mistake: The last time Doc tries the defibrillator on Norris, he uses it on the abdomen instead of on the chest.
Plot hole: Patrick Redfern needs someone to be in the boat with him to corroborate his and his wife's deception and be his alibi. That is whole point of his wife lying on beach - he gets this when taking Myra for a ride - but it was portrayed as a random ride she got, he did not intend or plan giving her a ride, he did not see her loitering near the boat, or even in view of him untying the boat. Even if that was intention in the story for him to be seen ready to cast off, the movie viewers did not see her viewing him, only her appearing just before he was set to cast off. Without someone with him the whole deception would not work, he would have no alibi, he could have killed Arlena unseen.
Continuity mistake: During the first rape scene, Carla is slapped in the face drawing blood and a pillow is placed over her face. After the scene, however, there is no blood on her face or the pillow.
Continuity mistake: Myra Bruhl is popping pills at the beginning of the movie; when she puts down the container, there's a white-labeled bottle on the bedstand that was not there before. (00:02:40)
Revealing mistake: Throughout the movie, practically every victim ends up with a broken, twisted neck. The "special effect" used for those deaths is...the actors wear their jackets backwards. It is made even more evident by the plot twist at the end.