Factual error: In the final scene there is a shot which shows a Land Rover in the distance. Land Rovers were not built until after World War 2.
Factual error: The film was released in 1958 but depicts a 1943 World War II mission in the Pacific by a US submarine capatain played by Clark Gable and his first officer played by Burt Lancaster. In two separate scenes the crew listens to a "Tokyo Rose" broadcast on radio and the background music is "Kiss Me Once, Kiss Me Twice, It's Been a Long Long Time" by Sammy Cahn and Jule Styne. The song was not released until September 1945, one month AFTER the end of the war in the Pacific in August 1945. The song was famous for celebrating returning troops and therefore also makes no sense in the context of 1943.
Other mistake: In the opening scene on the airplane, one of the men tells Lt. Cable, "The captain wants to know if you'd like to sit up in the cockpit with him?" Buzz Adams, the "captain" referenced in this line, is only a lieutenant. Admittedly, "captain" is a standard aviation term for the pilot of an aircraft, but it would be unlikely to be used in this situation, since the term has a specific military meaning.
Other mistake: Both sides (American & German) use the same tanks! The American tanks have white stars painted on them, while the German tanks bear white Maltese Crosses.
Revealing mistake: When Corporal Bin's group of men eventually arrive at Dunkirk, they speak to a soldier adding sandbags to a Vicker's gun position. Here (and later on in the film when it is shown firing) it has a blank-firing adaptor prominently fitted to the front. Additionally the belt of ammunition shown going through it is filled with blanks - no bullets project outside the other side to that of the actual cartridge cases.
Other mistake: In the opening scene, as Captain Glenn Ford is watching a Japanese freighter travel right to left through his submarine's periscope, the freighter is actually going backwards. The bow of the ship is on the right, not the left, as it should be.
Other mistake: During training, when the men are running and the two British men come out of the bushes then shoot at the ground, there are 3 rows of bullets going past but only 2 guns being fired.
Continuity mistake: When Diestl and Hardenburg attempt their wild dash to the coast on the motorcycle, they pass the same wrecked Kubelwagen three times.
Revealing mistake: Whenever our heroes are shown firing the Vickers guns on or off the vehicles, they are very obviously fitted with blank-fire adapters to the front. The belts of ammunition for them throughout the film are also blanks - the cases are visible, but the bullets that should project out the other side of the belt are not present.
Continuity mistake: When the Bayonet is thrown at Strong, it is thrown with the butt of the rifle facing left, but is caught with the butt facing right. (00:41:00)
Factual error: The film is set during WW2, but when Pitkin drops Judy off at the station, the locomotive bears the lion and wheel emblem of 'British Railways', which was formed in 1948. Likewise the wagons in the railway yard are to a British Railways standard design and are out of period for WW2.
Factual error: The movie's title "The Inn Of The Sixth Happiness" is the name of the mission station that Gladys Aylward/Ingrid Bergman sets up. In reality this was called "The Inn Of The Eighth Happiness." Numerology is popular in China, where eight is regarded as a particularly auspicious number. Apparently the film company thought "sixth" had a better ring to it than "eighth." In the movie it is explained that there are six levels of happiness. This is not a Chinese belief and seems to have been invented for the movie.