In several interior scenes, Rock Hudson's hair style and sideburns are drastically shorter than in the rest of the film. This seems to give credence to the rumors of studio interference and the necessity of re-shoots.
The planes' guns (both Allied and German) fire too quickly for the bullets to pass between the planes' propellers.
When the telephone rings in the film, it is with the British double ring rather than the single ring that would be heard in France.
Public address (PA) announcements are heard when Lily Smith and Colonel Kurt Von Ruger enter the train station at about 2:00. However, PA systems were invented in the US in 1915 and weren't commercially available until the 1920s - several years after the movie's 1918 setting.