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.
Eye of the Needle (1981)
1 revealing mistake
Directed by: Richard Marquand
Starring: Donald Sutherland, Kate Nelligan, Stephen MacKenna, Philip Martin Brown
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.