Revealing mistake: When Bela Lugosi walks off screen, it's obviously a freeze frame. The swaying tree stops moving and even though he was hit by a car, his shadow is still visible. (00:07:55)
Factual error: As Frederick is explaining to his guests how to use their .45 automatics, he fires one round at a near-by vase, shattering it. However, it leaves no bullet hole in the wall directly behind the vase.
Visible crew/equipment: Michael Pate is walking down the street in this western and as he passes a shop window you can see all the camera crew, the director, the arc lights etc.
Continuity mistake: After Sir Hugo has stabbed the girl on the moor, no blood is visible on the knife he used, however after he drops it (because he is attacked by a hound) blood has now appeared on it.
Other mistake: When the Behemoth swims up the river & capsizes the ferry, a static model is used that bears no resemblance to the animation model seen throughout the film.
Continuity mistake: The doctor just killed Mr. Fleming. Cut to Agnes Moorehead playing cards; She is casting no shadow on the desk from the side view, but in the view with the camera in front of her, she is projecting a shadow. (00:11:35)
Factual error: Egyptian death masks eyes were open not closed. It was part of their beliefs regarding the afterlife.
Revealing mistake: Near the beginning, when police Lieutenant Rowan is inquiring on the death of Ken Drake, there are only 4 men in the room, all facing one another in a wide shot. The camera cuts to a close-up of Dr. Bradford for a couple of seconds, and there is a distinct puff of cigarette smoke passing behind him, but none of the characters in this scene are smoking. Lieutenant Rowan does smoke later in the film (in fact, he's the only character who smokes), but he's not smoking in this early scene. Apparently, the close-up of Dr. Bradford was borrowed from omitted footage in which Lieutenant Rowan was smoking.
Continuity mistake: Christopher Lee looks outside his useless nephew's room and spots two hot Swedish girls. Renato Rascel has a bottle of wine in hand and walks towards him, but the bottle is gone when he tries to stop him. (00:23:40)
Other mistake: After the intro, one girl in the bar peeks from the window and goes; "Say, here comes Gordy and Jennie." And indeed the duo is getting off the car and joins them. The problem is, the window is on the opposite side of the entrance and there's diegetic music playing throughout the scene. In other words, she saw them parking to the left, but in a split second they are coming through a door on the right. (00:02:40)
Other mistake: A Telechron electric clock on the fireplace mantel keeps time even though the power has failed.