Factual error: During the war games they have Jefferson pose as a major in order to facilitate the commandeering of the ambulance and the jeep. Regardless of their personal feelings in the matter the ambulance crew and the driver and troops in the jeep would be perfectly aware that no black man would ever be promoted to a position of authority in an otherwise all white command. We may find it repugnant today but the US Army was rigidly segregated during World War 2 - and it stayed that way until 1948. Jefferson may have been inducted into a special unit like the Dirty Dozen but considering that the future of the entire mission is riding on their success at the games, throwing it all away like that makes no sense at all.
Factual error: The raid on the chateau takes place during the evening before the D-Day landings, in Rennes in Brittany, almost exactly halfway between Cherbourg and the Normandy beaches - both thought highly likely to be the site of the Allied landings. On the evening before the landings - the night of the raid - the area was heavily bombed by the RAF and dummy parachutists and radar-jamming devices were dropped into key areas, many close to Rennes itself. One dummy parachute drop succeeded in drawing an entire German infantry regiment away from its position just six miles from the Normandy landing beaches! The Germans knew something was happening and the landings were imminent - they just didn't know exactly where they would happen. Given all of this it is absurd to think that so many high ranking German officers would be enjoying themselves in a glorified brothel. They would be with their units preparing a counterattack, or at the very least on their way back.
Factual error: When Resiman interviews him in prison, Franko is chewing gum. (In several shots you see it is chewing gum and not tobacco). First, during heavy rationing in the UK in World War 2 chewing gum was a rare and expensive treat. A military prisoner would not be able to obtain it for love or money. Secondly, US military prisoners were never, ever allowed chewing gum - it can be (and has been) used to jam locks.
Continuity mistake: In the wargames scene, right after the group comes down the hill with the live artillery fire, they change arm bands from blue to red. In the next scene when they pick up the stolen jeep, everyone is wearing blue arm bands again (when they went over the hill, they were in red territory so would not have changed back to blue while walking to meet the jeep). Then when they crash the jeep, they are back in the red arm bands for the remainder of the scene.
Continuity mistake: As the Dirty Dozen ready for the wargames, they are in a truck passing and saluting a line of American light tanks in closeup. Immediately after turning left into a circular driveway and stopping, in a long shot the tanks have quickly turned into a long line of Army trucks. There was no time for the tank convoy to drive off and have a convoy of trucks enter the scene.
Continuity mistake: Lee Marvin is shot in the right shoulder while driving the halftrack away from the bombed Nazi Villa. He places his left hand over the wound which becomes almost totally covered in blood. The camera pans off the truck and then back, and his left hand, still over the wound, is clean.
Factual error: The film opens with a US soldier being hanged, the lever is operated by a US Noncom. Wrong. The US forces were allowed to hang their own on British soil, but only if a British executioner was used. The executioner would have been our own Mr Pierrepoint.
Revealing mistake: When the tank/truck driven by Marvin is crossing the stone bridge at the end, they are met by a small German gun vehicle, which they run off of the bridge. The camera angle changes to below the bridge and you see the German vehicle fall from the side of the bridge into the water below. If you look at the bridge material that shatters you can see pieces of wood and hollow areas where obviously it's not real stone. (02:25:00 - 02:30:00)
Continuity mistake: At the parachute training school,the dirty dozen have fairly thick beards. Later when Col. Breed pays a visit to their camp, their beards are just stubble.
Factual error: Col. Everett Dasher Breed is wearing Ray Ban Way Farer sunglasses. They were not designed until 1952.
Continuity mistake: Outside the chateau Donald Sutherland is met by the German soldier who uses his cigarette to lite his own. The German soldier is then shown walking away with the camera facing him and Sutherland exhaling smoke. The next immediate cut shows a front shot of Sutherland but he's not exhaling smoke. (02:00:00 - 02:05:00)
Continuity mistake: Near the end, when everyone is getting ready to escape in the German halftrack, the Major notices that the vehicle is attached to an artillery gun. He looks back over his left shoulder but in the next shot, a split second later, he looks over his right.
Other mistake: There is a lot of shooting with automatic weapons, and a lot of Germans fall - but there is a remarkable lack of bullet holes anywhere; even in such tight quarters as the bell tower, where bullet holes should have appeared.