Continuity mistake: After Melanie signs the card to Cathy she never puts her left glove back on. We're able to see her drive down uninterrupted to the boat and she never puts it on, but when she walks out of the car to the boat it's back on.
Continuity mistake: After Melanie has been bandaged up after her attack, she sits up on the sofa, and you can see two large bloody scratches on her cheek. These scratches disappear when Mitch and Lydia walk Melanie outside to the car.
Continuity mistake: When Tippi Hedren is having her head wound cleaned in the restaurant, Rod Taylor is wiping her blood off with a tissue but it does not actually come off; in the next shot, however, there is clearly no blood on her head.