Revealing mistake: Billy, a young village boy, wants to join the army and fight in the war, but he is under the 18 y/o age requirement. He's supposed to be about 15 or 16 years old, but it's quite obvious that the actor playing him is in his early 20s and definitely looks old enough to join.
Deliberate mistake: When "Henry Faber" discovers the phony airfield, several helicopters with Army markings are among the aircraft visible. Although the Army did use helicopters experimentally during WWII, those used bore no resemblance to those shown in the film.
Continuity mistake: When Kate breaks her glass, hurting herself, Donald wraps his hanky around her hand, to try and stop the bleeding, but when they start kissing and hugging, there is no blood.
Visible crew/equipment: After Faber has hitched a ride (when his motorcycle failed,) the driver pulls over to let him out. As the car pulls away from Faber you can see a crew member in a blue vest type coat reflected in the passing car window.
Plot hole: David couldn't have seen Sutherland's photos because Sutherland hadn't had time to develop them yet. Even if he did, it would have been difficult to find a lab that wouldn't turn him in as a spy. Overall, it would have been much safer to have the film processed in Germany.
