Continuity mistake: Immediately after the (Dali) dream sequence, when Dr. Brulov and Constance are reviewing Constance's notes, Dr. Brulov draws a pencil out of his coat pocket. Hitchcock then cuts to a close-up, and Dr. Brulov repeats the motion.
Continuity mistake: When Lombard tells Vera "Don't you think it's strange that there's never a third person present when anything unpleasant occurs?" his cigarette is in the left hand, and switches instantly to the right in the next shot. (01:03:35)
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)
Continuity mistake: After the scene with Junior that busted him as the night kitchen raider, Dick is in his office. Pat Patton tells him about his findings going through the victim's belongings. He leans forward with his left arm, and Dick at the same time pulls his own arm back away from the desk. Next cut and Dick has both hands on the desk and Pat just his right arm on it. (00:12:35)