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.
Factual error: Col. Everett Dasher Breed is wearing Ray Ban Way Farer sunglasses. They were not designed until 1952.
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.
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.
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.
Continuity mistake: At the very end, Jefferson is shot after dropping grenades into the vent pipes. He lands face down with his right arm bent and his right hand right next to his helmet. The camera cuts to those left on the truck. When it shows Jefferson again, his hand is now about a foot away from his helmet.
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.
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.
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.
Continuity mistake: When Waterslaw and Jefferson each shoot a soldier through a window, no glass seems to break until we see each soldier fall out through the glass.
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.