(at around 1h 30 mins) During the "war games" sequence, some of the "Dozen" are shown to exchange their Blue Army armbands for the red ones worn by the opposing forces. But for the next few minutes of the film, they are still wearing their blue ones.
(at around 16 mins) When Franko refuses to participate in the close order drill, the direction he is facing changes between ground level view and above ground view.
(at around 2h 25 mins) At the end when Reisman is driving over the bridge, another vehicle of Germans comes from the opposite direction. He lifts his machine gun over the frame of the windshield. The camera cuts to another angle, and from behind Reisman he is still holding over the windshield. Another cut to the front of his vehicle shows he has not yet raised the gun at all, but it is then that he lifts it from the seat.
(at around 1h) An airplane in the sky during the "fake General's inspection" scene disappears a frame later.
(at around 1h 50 mins) After Wladislaw spills the ink on the guestbook at the château, the German corporal tells them again to sign the guestbook.
(at around 1h 35 mins) During WWII no Black man could have attained the rank of major in an all white outfit so he would not have fooled the doctor or ambulance driver. Someone else should have been chosen as the leader of that unit for it to work.
Although US military personnel were executed on British soil during WW2, the hanging sequence in this film shows US Military Policemen carrying out the execution. In reality, they were not legally allowed to do this. Instead, the hangings were carried out by British hangmen such as Albert Pierrepoint, with American personnel acting only as official witnesses.
(at around 56 mins) The 101st Airborne's band (when playing during the visit by the "General") was playing "Semper Fidelis", the official march of the United States Marine Corps, composed by the United States Marine Band's first & most famous conductor, John Philip Sousa. This piece of music would not be something a United States Army band would play.
(at around 1h 45 mins) When the unit parachutes behind enemy lines for the mission, the major is wearing a helmet with a bright, shiny major insignia on the front. That never would have been worn into combat like that, as it would give the enemy a great target to take out the officers and thereby disrupt morale.
In the US Field Artillery, Batteries are referred to by letter not number, Headquarters and Service Battery, Battery A, B Battery, and C Battery.
The U.S. Armed Services were not desegregated until 1948, yet outside the core group, nobody even blinks at the sight of an African-American in the same squad as white soldiers.
Wires visible on a flare during the attack on the German compound.
(at around 1h 6 mins) When going through the airborne school, the static lines are not connect to the parachute and can be seen when some jump. The cloth ribbon they used for the shot would also not work with as too thin.
Major Riesman's men deliberately kill large numbers of innocent female civilians, namely the wives and mistresses of the German officers, in the destruction of the chateau. That would constitute a war crime under recognized rules of engagement that would justify retaliation on the past of the Germans against Allied women POW's.
After the General decides that the DD will be tested against Colonel Breed's Red forces, the entire dozen drive to the test site wearing Red armbands, and then immediately change to Blue armbands. This is nonsense. They would never worn Red armbands until subterfuge made it was necessary, as they wouldn't want it known they had Red armbands.
(at around 1h 10 mins) The clothing and hairstyles worn by the women in the "graduation ball" are 1967 hairstyles. The film is set in 1944.
While almost all of the male hairstyles and uniforms look like they could be from the 1940's, not one of the female actors has a hairstyle or is dressed as they would've been in 1944. They scream 1960's with a vengeance.
General Worden's barracks cap has gold braid on the visor, incorrect for the World War II period. Douglas MacArthur was the only U.S. Army general who wore braid on his cap, but this was actually his Philippine Field Marshal's cap and not U.S. issue.
(at around 1h 40 mins) In the dinner scene, at the end of the training and just before the Dozen are in the plane, one of the characters shoots a picture with a Kodak Tourist Flash camera not produced until 1951.
(at around 2h 5 mins) In the chateau scene, the German girl has a 1967 hairstyle and clothing.
(at around 1h 5 mins) When Wladislaw is beat up in the latrine, the men in the truck ask what happened, but when Franko sarcastically says, "Three wise men, trust the major", the last three words do not match his mouth. It appears he says "Trust Reisman".
(at around 1h 7 mins) Jiminez plays guitar while singing The Bramble Bush. After a pause, his hands continue strumming but the guitar makes no sound.
(at around 1h 20 mins) After the scene where the Dirty Dozen remove the weapons from the regular soldiers they can be heard laughing and taunting the Colonel as Major Reisman watches. However, their mouths are closed.
It was madness for Major Reisman to enter the Chateau disguised as a German officer with no knowledge of German. He would have been found out immediately if approached by the German officers to engage in conversation or to identify himself. Conveniently, the sole German who speaks to him is dead drunk and Reisman is able to brush him off by just nodding his head.
There appears to be no military reason for Jefferson to be ordered not to blow up the air vents until the German half track reaches the bridge, other than to make sure Jefferson gets killed.
As Maggott was obviously insane he would never have been allowed on the mission. In fact it is likely he would have been hanged before they went to France.
The château is basically a glorified brothel for officers without a significant protection detail or specially secured storage facilities. So there is no reason for there to be massive amounts of ammonium picrate (aka Dunnite or Explosive D) in the cellar of the château.
It is clear that Major Riesman, who impersonates a German officer in the chateau, does not speak German. When confronted by a drunken German officer, he merely nods his head politely and is left conveniently alone. The game would have been up if some sober German officers tried to engage him in conversation or did not recognize him (as would seem quite likely) and confronted him as to his identity. Accordingly, his presence in the chateau was foolhardy and placed the entire mission in jeopardy.
After Pinkley's pseudo general act, Col. Breed angrily calls his two sergeants "Clayton, Blake!" As they respond, for a few seconds we see that one of them is chewing gum. It's very unlikely that a completely by-the-book officer like Col. Breed would allow his men to chew gum, especially during an inspection.
(at around 1h 50 mins) Wladislaw admits that he couldn't understand the German officers who greet him and Reisman in the chateau, but in the dialogue in question, the officer says in quite intelligible German, "Good evening, sir. Lovely weather we are having, no?" Considering how well he understands other Germans in the chateau, it seems unlikely he would have trouble with this clearly spoken line. In reality, it was probably easier for the actor playing the German to deliver the line in clearly-spoken German than to approximate a harder to understand dialect.
(at around 15 mins) After Sgt. Bowren dubs the "Dirty Dozen" with their titular nickname, he orders, "Dress right, dress." The men space off with left hand on belt and elbow extended to the side, which is "close interval dress". At the sergeant's order, they should have spaced off with the left arm extended at shoulder level.
In one scene Posey says "Captain Kinder figures he can teach me letters" implying that he is illiterate. However, in an earlier scene, he is seen at least twice holding a clipboard, and once writing on it. While it's true that some illiterate people can actually read and write numbers, it's very unlikely that Posey would be given that kind of job and responsibility in the camp, which obviously requires checking or writing numbers.
Wladislaw tells Franco that he (Franco) can't go with them on the advance team to Breed's headquarters during the wargames, stating that Franco and Pinkley were the only ones that Breed had gotten a good look at. Wladislaw also shouldn't have gone, since Breed's two men had jumped him in the latrine at the jump training. Breed, knowing the Dozen were going to try for his headquarters, should have had those two men on guard since they had gotten close looks at several of the Dozen, Wladislaw included.