Trivia: Not really a mistake, given the need for drama in a war film, but the mission takes place the evening before D-Day, meaning the Allies had complete air supremacy, and knew the exact time, date and place of the meeting of the German officers, this mission would never have taken place. The brightly lit chateau would have been flattened by a squadron of heavy bombers. Lee Marvin, an experienced combat veteran, pointed this out to the producers and was told to keep his opinions to himself.
Trivia: We never actually see Posey (Clint Walker) die on film. We know where he is during the mission and we only saw his partner Bravos get shot but not Posey. I always have this funny feeling that he may have survived and the surviving members (Lee Marvin, Charles Bronson, and Richard Jaeckel) of the Dozen forgot about him.
Trivia: Notice how the only survivor (Bronson) isn't a "real" Criminal (rapist, murderer etc) but shot a soldier running off with the unit's medical supplies. The Hollywood Code of the time couldn't show criminals "getting away with it."