Continuity mistake: When Dana Andrews and Jeane Craine are on the roller coaster, as it goes down the first dip, she stand up and he yells at her to sit down, but his mouth never moves. Also, the positions of the riders change from scene to scene.
Revealing mistake: The "hand" holding the gun rotates 180 degrees while the shooter's point of view does not move. A real hand can't reproduce that motion without moving extremely close to the face.
Continuity mistake: Even though the heavy rain had only just stopped, as Laura sits alone on the bench, the pavements around her are completely dry.
Factual error: This is not an error in the film itself, but in some supplementary material on the DVD, in which Margot FitzSimons, the actress who plays Bridie, is identified as the sister of Maureen O'Sullivan. She is in fact the sister of Maureen O'Hara.
Continuity mistake: Doctor Leo sits at the desk and begins writing a letter to Alec Ritchfield. "My dear Alec", it starts. Fade to the finished letter, but you can easily tell that it's not the same letter, the handwriting being different - look at the little flourish below the M, not to mention the position of the text in relation with the Fog Island stamp. (00:08:25)