Factual error: The crashed US spacecraft from which the monster emerges is obviously much too small to have carried a human being into space.
Continuity mistake: In the film the Eye Creatures have landed their spaceship in a field during the night and the Army has been called in to survey the spaceship. A soldier gets on top of the space ship and starts using a welding unit to try and open it and it's bright and clear out. The temperature of the welding ignites a fuse on the spaceship which causes the ship to explode. When the scene shows the spaceship blowing up it's now pitch black outside.
Visible crew/equipment: In the final scene where Frankenstein is fighting the Spacemonster on the spaceship, there are upshots of the monster attacking to make him look larger, but looking right behind his head you can see a ceiling mounted fluorescent light and electrical conduit; neither of which would have been on a spaceship.
Other mistake: In the opening credits, the composer of the movie score is credited as "Elizabeth Lutyens", with a Z. Her proper name was Elisabeth, with an S. (00:00:50)
Other mistake: Dean Jones and Connie Stevens are riding in a gondola of a skyway ride at a seaside amusement park. The gondola they are in is obviously on a studio soundstage. It is much larger than the gondolas shown in the rear-screen projection. In addition, there are no riders in any of the other gondolas and the gears and cables of the ride make absolutely no sound.