Continuity mistake: Barbara Rush aims a death ray to the right and above of Carlson. The scene changes with Carlson ducking and now the ray is to the left of him at waist level. The ray hits the rocks behind Carlson for several seconds, but it doesn't start breaking the rocks up on the wall until Barbara swings the ray from left to right.
Continuity mistake: The opening narration says the season is late spring, or otherwise a warm season of the year in Arizona, yet many people are wearing wool suits, coats and long-sleeved shirts during the day. At one point, the sheriff's dialogue even discloses that it is 92 degrees (and no air conditioning).
Visible crew/equipment: Opening credits (also a few minutes in) has a fiery spaceship crashing into the camera for this was a 3D movie. What's interesting is that a mirror can be seen on the left portion of the screen just before the collision. This was apparent at the theatres and on the VHS tape release. For some reason, the DVD release has it cropped out and now the spaceship is off-centered at the beginning (fortunately the bonus material still shows it).
Visible crew/equipment: The alien takes a stroll through the Arizona desert after crashing. The animals are frightened and run away with the exception of the owl. The bird has a string tied to its left leg and someone off camera yanks it, and the bird screeches and does a complete flip while flapping its wings. It looks hilarious - poor owl.
Revealing mistake: The old telephone repairman is shot at by the posse as he is driving his repair truck. The bullets hitting the windshield leave black marks, but it is hard to tell if it is a special type of glass or paint balls when the "bullets" hit. Why? There are NO holes in the windshield as you can see in the close-up of the actor in the truck face-on.
Plot hole: An alien transforms into Russell Johnson with clothes on. Yet at various times through the movie, the aliens have to go to their victims home to get their clothes. I guess a naked Russell Johnson (the professor on Gilligan's Island) would have been too much for 1953 audiences. (The aliens in this movie were highly neurotic about their looks.)