Continuity mistake: In the scene where Ursula Andress confronts Marcello Mastroianni, he reaches into an ice bucket where you see a very small pistol (PPK). When he points it at her, it changes into (I think) a Colt 45. It then, in another angle changes again into (I think) a Mauser Parabellum... Mistakes can be made if filming covers many days on the same scene, but when the dialogue mentions how he can hide a tiny pistol in the ice bucket.....
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.
Revealing mistake: When the gang tries the growth formula in the theatre, they quickly grow out of all their clothes until they are naked. But the scrunchy in Jean's pony tail is still in place and shows no signs of being stretched, even if the pony tail now is 20 times bigger.
Revealing mistake: During practically every scene with Dr. Nadir, the prosthetics to make his cranium look more 'alien' are visible. In particular, when the Queen realises he's been shooting down spaceships and not missiles and calls him a 'fool', at the back of the neck the line where the concealing make-up ends is evident. Throughout the rest of the movie the other Martians have similar if not even more obvious bald caps. (00:17:45)