Continuity mistake: When the Disco Volante separates to escape the cruisers firing on it, the "cocoon" is seen facing the pursuing ships stern-first. When the shot changes to show the gunners firing on the ships, the back half of the "cocoon" has turned completely around. The gunners are firing from the separated mid-section of the Volante, and the stern of the "cocoon" is now facing away from the cruisers, making it easy to see which parts of the scene were filmed live and which were done in the studio.
Continuity mistake: When the Leslie Special is rolled out to be presented to the Great Leslie the red carpet in the front view starts at the building alignment. The aerial shot as seen from Professor Fate shows a wide gap between the building alignment and the red carpet.
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 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: 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 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: When the train is taking water it is sitting on a far right spur line, but when it leaves it is on the main line.
Continuity mistake: In the scene when Richard Harris and Kirk Douglas parachute back into Norway, there is a white parachute blowing in the wind. Kirk Douglas goes to collect it, but there are footprints already in the snow where they have previously filmed a retake. (00:27:00)