Just minutes into the movie when Captain Gabriel Jackson is traveling on the train. The train seats are lined in British Rail Second Class Moquette material used to line the seats of carriages in the 1970s. British Rail didn't exist during the Second World War.
Just seconds into the film, a German MG-42 machine gun is shown firing on British soldiers. While in reality the MG-42 fired between 900-1500 rounds per minute, the sound effects indicate a much slower rate of fire, less than half of the true rate.
The ribbons on the British traitor Major's uniform are US Marine Corps awards - Joint Service Commendation Medal; Good Conduct Medal; & Marine Corps Expeditionary Medal. At the end of the movie (while on the train), Capt. Jackson is wearing the same ribbons on his uniform. Those ribbons change when he approaches Mrs. McBain.
At 28 min in, all of the crew are using Mosin-Negant rifles. While the Nazis captured Mosins' in the Soviet Union and were put into service on the Eastern Front, they were not put into service in France.
All weapons appear to be non-firing replicas. Especially visible are the Thompson sub machine guns- in many cases the bolts are forward, meaning the weapon is closed on an empty chamber. There is no visible movement of bolts, charging handles, ejection of spent cartridges, etc.
In the first few minutes, when the plan is outlined to the commandos, the commandos are bare-headed but salute the officers. They would not salute if not wearing any kind of uniform headgear.
While on the train the Ranger Capt in not wearing his Triangle Ranger Patch.