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: Near the beginning, Lt. Col. Daniel Kiley is in a scout plane and flies over Col. Martin Hessler, who is in a German staff car below. Kiley tells the pilot to "rev the engine" just above the car to entice Hessler to look up at them, so he can get a good photograph of his face. The plan works, and the ground level movie camera shows Hessler looking up. When the "photo from the air" is later developed, it shows Hessler looking up from the ground level camera's perspective, eyes focused about 30 degrees behind the airplane and giving a profile of Hessler.
Continuity mistake: When they are all lined up for their medical inspection at the beginning of the film, the sun is coming up behind the hut casting a shadow in the front. When the Sargeant Major is then shown shouting from the hut, "I want this one (Bartlet) scrubbed with a yard broom." You can now see by the shadows that the sun is shinning directly at the hut. This would be impossible as there is only a few seconds between shots for this change to happen.
Continuity mistake: Near the beginning when Lee Van Cleef looks at the the wanted poster on the ticket booth at Tucumcari (just after getting off the train), there are two extra zero's drawn onto the poster (as added by the villain). Van Cleef then takes this poster away with him, and slots it under the door of a hotel room on a later scene - notice that the additional zero's have since disappeared from the same poster. (00:06:30 - 00:09:20)
Continuity mistake: The city railroad scenes were filmed in Spain, and many of the forest railroad scenes were filmed in Finland; in both of those countries, most of the railroad track is broad gauge (the rails are more than 5 feet apart). The plains and mountains railroad scenes were largely shot in Canada, where the rails are "standard gauge": 4' 8 & 1/2" apart. The rails can be seen to jump between far apart and closer together more than once as the movie progresses.
Continuity mistake: When the German colonel crash lands in the Channel, he first hangs directly under his upside-down plane before he has to let go, and the plane zooms in a straight line away from him. But as he resurfaces in the foreground, the plane comes in from the background's left before landing in the water.
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.