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.....
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.
Continuity mistake: When one of the kids break into the laboratory, everyone else is partying in the garden. In the outside shots, it is dark, but in all the shots from inside the lab you can see through the windows that it is daylight outside.