Deliberate mistake: For all exterior shots on the surface of Mars, the crew's "pressurized" space helmets have no face-plates (their faces are fully exposed to the hostile Martian environment). The transparent face-plates they intended to use reflected far too much studio set lighting, such that the glare would have obscured the actors' faces. On a $200,000 budget and a 10-day production deadline, they simply scrapped the face-plates and shot the scenes with wide-open helmets.
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: Bugs is wearing a bow-tie when he is sitting in his cot, but when Mama Ape picks him up, the tie has gone.
Factual error: Karnak was an Egyptian city, not one of their gods.
Continuity mistake: When Sylvester is testing out the artificial storm cloud he has created, he is holding a dropper with his right hand. But when we see Sylvester in the next shot, the dropper has gone.
Continuity mistake: The car is out of gas, and Sylva Koscina gets impatient. As the shot changes, Walter Chiari's arm is on top of the car door (it wasn't in the first shot) and Dorian Gray crosses again her arms on top of the bag - assuming the same pose she already had. (00:17:00)
Continuity mistake: Sam Boone walks Ben Brigade to the horses holding his rifle, and they have a bit of a chat by and about the captive Billy. During this scene, the handkerchief at Randolph Scott's neck waves like mad in the wind over his shoulder under one camera angle, and it is still and in front of the chest in the other. (00:09:30)
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)
Continuity mistake: When Elvis hands a pair of binoculars to his pappy, you can see there is a swarm of flies buzzing around Pappy, but in the next shot the flies have disappeared.
Continuity mistake: When Richard Hannay jumps from the Sheriff's window into a passing truck it is full of bales of hay. The next time we see him in the truck it is full of sheep and the bales of hay have vanished.