The hand-held device that controls the mechanical bed has a red button on top and a green button on bottom. But later, when the killer is pressing the controls, the stop and start buttons are in opposite positions.
While watching film clips,Dr. Death's mask is up/down between shots.
While watching film clips, Price dressed as Dr. Death is first shown with his mask up and then down and then shown pulling the mask down.
Julia's dead body is breathing heavily after Paul Toombes discovers her.
When the police are examining the hanged woman's body, the camera shows her face reflected in a piece of gym equipment. Her eyes clearly move as the camera tilts up.
When Paul is searching the house at night, the light of his candle is actually a spotlight, and it is does not keep very good pace with his movements.