Factual error: After the birth of the baby, James Stewart and "Doc" Witherspoon are discussing the course of the war. "Doc" relates that he has always lived in Virginia and that his son died at Gettysburg at Little Round Top. No Virginia regiments fought at Little Round Top - only Alabama, Texas, Georgia, and Arkansas regiments.
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 scene where the Beatles are in bed sleeping, and Ringo is getting pulled out of bed by the ring, he falls out of bed, and brings the blankets with him. In the next shot however, the bed clothes are back on his bed.
Factual error: None of the German tanks are accurate representations of German WWII types. They are all post war US variants. ie. M47 Patton Tanks. [Still a mistake, but an elaboration: This is a common problem in depicting German tanks: few, if any, survived the war. The few museum examples wouldn't be released for movie work, and certainly WHERE, some 20 years after the war, would one find a brigade's worth of running Tiger II tanks? Never mind that Kampgruppe Pieper, which Hessler's command is loosely based on, actually used Mark IV and Panther tanks. There was one King Tiger battalion, and it was in reserve well behind KG Peiper. The M47s versus the M24 Chafees (mistakenly called "Shermans" at times) is a fairly accurate comparison of the respective sizes, firepower, and other combat capabilities of the respective tanks. Some other movies, like "Kelly's Heroes", have their "Tigers" mocked-up with sheet metal, wire mesh, and sacking over a different type of tank (in that case, T-34/85s of the Yugoslav army are mocked up as Tiger Is, a fairly decent job).]
Continuity mistake: In the last scene, 2nd Lt. Blair is holding an army carbine on which the barrel is noticeably bent downward.
Factual error: Terry-Thomas's plane gets stuck between the carriages of a Paris-bound train (in fact filmed on the single-line track between Bedford and Hitchin). Just before the train plus plane goes into a tunnel you can see the cooling towers of Bedford's Goldington Power Station (circa 1960) behind TT's head. (01:58:20)
Revealing mistake: After Manco beats up Cavanaugh and places Cavanaugh's back up against the bar, while Manco faces him, three of Cavanaugh's henchmen appear at the entrance of the bar demanding that Cavanaugh be released. Manco turns around and kills all of them with his pistol, while Cavanaugh falls to the floor crawling toward his gun. Manco is walking toward the entrance of the saloon with his back to Cavanaugh. Manco draws his gun and kills Cavanaugh without looking at him. If you look closely at the tip end of the pistol, you can see that the only thing that Manco would have killed was the saloon floor. (00:17:00)
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.
Audio problem: In the scene when Carole catches Ms. Lefavre in Michael's apartment, she (Ms. Lefavre) tries to cover by pretending to be Michael's maid. As she moves about, cleaning various items, she talks about all the work she has to do, etc., etc. However, throughout this entire sequence her mouth never moves or so much as opens.
Factual error: The daylight air-raid on the Norwegian Heavy Water plant in 1942 was done by Boeing B17G bombers, with the distinctive chin gun turrets. These planes did not come into service until 1944, when they were introduced to counter the head-on attacks by German fighters.
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.