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.
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: 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.
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.