Revealing mistake: When the UFO makes the huge flash that knocks everyone in the cemetery down, one of the police officers kicks a tombstone as he falls and it wobbles back and forth. (00:13:35)
Character mistake: In the scenes in San Diego, all the scenes outside the sub are taking place during daylight hours. Immediately after the crewman relays the message about the mysterious telegraph transmission, the scene shifts to the sub. The commander asks what time it is in Melbourne and is told it is 1500 hours (3PM). But if it were that time in Melbourne, it would be 10PM in San Diego, and thus not during daylight hours. (01:33:30 - 01:38:05)
Factual error: Iceland sure looks a lot like the California desert.
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.
Other mistake: A Telechron electric clock on the fireplace mantel keeps time even though the power has failed.
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: When Holmes kills the guineapig-human hybrid, the entire scene is ridiculous. In the first shot, the guineapig has tiny human hands that look to be about a half-inch at most. Holmes then steps on it, and in that shot it appears to be a different type of rodent entirely. Then we see the human hands under his shoe flexing in pain, and they look to be significantly bigger than before based on the scale of his shoe... at least 5X bigger than they were.
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)