Goofs

A match is struck to light up the guardroom. It stays alight for almost 20 seconds and barely burns down at all.

When the Avro York lands in Africa, you can see that its civil registration has been painted over under the port wing.

In the movie M. E. Clifton James is attached to Montgomery's personal staff and assumes the role of Corporal Walker, a driver, in order for him to study Montgomery at close quarters to learn his mannerisms and speech. In reality, James was a correspondent.

When Monty's double's Avro York heads across to Africa, and German fighters are seen scrambling to attack it, the next seen are RAF Spitfires scrambling to defend it. The film shown is newsreel footage from the Battle of Britain era, with the famous bell, inscribed, "Ring the bell, and run like hell".

Many of the background vehicles and fashions are completely incorrect for a film that is set in 1944. This is very apparent during exterior shots in London, and especially so in the last scene when Major Harvey, Colonel Logan and Lieutenant Clifton-James exit the theatre. A lot of the cars are distinctly 1950's era and passers-by are wearing late 1950's fashion. This is most noticeable with the young man in the suit and tie that Clifton-James bumps into, and also the man to the right of the screen wearing the long coat when the camera zooms out for the final closing shot.

When Major Harvey (John Mills) and Butterfield--his spur-of-the-moment "recruit"--discover the dead bodies of the British security guards shot by German infiltrators outside the building where Monty's double is residing, Harvey unwisely only grabs one of the security guards' machine guns for himself in the hope of rescuing James. What he should have done was grab another machine gun and hand it to Butterfield in order to improve their presumed strategic advantage.

Soldiers refer to the Queen's Regulations, whereas at this time they should be the King's Regulations (King George VI).

When recruiting James, Major Harvey tells him he's going to impersonate Montgomery "Before D-Day." This term would have been meaningless at the time. D-Day (together H-Hour) was a term used in military planning, and meant the day when some plan would be put into action. The usage had started during WW1 (the British adapting the French term Jour-J), and every operation during WW2 would have had a D-Day. It was only after the war that the term came to specifically refer to the Normandy landings.

When M.E. Clifton James is taken by Harvey to meet Field Marshal Montgomery to gain his approval - and to overcome his nerves - James is wearing the service dress uniform of a Lieutenant in the Royal Army Pay Corps (which is correct). However, his cap badge bears the post-1953 Queen's Crown rather than the 1910-52 King's Crown which would be correct for the time of the action in the film, 1944.

Gibraltar is seen to have prominent fluorescent lighting strung across the street. This wouldn't have existed at the time of the action.

Aerosol sprays containing hair dyes or paint weren't available during World War II. Also, the aerosol can used is a modern design that wasn't developed until after the war.

The film opens as John Mills' character steps ashore behind a caption "The South Coast of England". Within minutes he is seen arriving at, and subsequently being tailed through, London's Liverpool Street Station... which serves the East of England and is not accessible by train from the south coast.