Approximately one hour into the film, when Colonel Thursday and Captain York prepare to leave the fort to protect the wagon-team led by 2nd Lieutenant Michael O'Rourke from an anticipated Indian attack, the class-conscious Thursday criticizes York's soldiers for their sloppy uniforms, pointedly telling York himself that York's hat should be creased "like a fedora." The action of "Fort Apache" takes place during the lifetime of Cochise, the famous Apache chief who died in 1874. The word "fedora" does not enter the language until 1882, when the hat worn by Sarah Bernhardt as Princess Fedora in Victorien Sardou's hit play "Fedora" became the rage of the fashion world. Thursday's use of the word is an anachronism.
When the new recruits are assembled for the second time (in uniform) the two closest to the camera appear to be wearing denim jeans or Levis. Levis were not available to the U.S. Cavalry at the time of the film's period in history.
When Col. Thursday leads a company "at a striking distance" from a repair patrol lead by Lt. O'Rourke. Sgt. Mulcahey is part of O'Rourke's patrol, but when the action shifts to show Col. Thursday's company in a saber charge against the Apache's there is a clear shot of Sgt. Mulcahey riding next to the flag bearer with his saber drawn. Note: this scene was duplicated from Col. Thursday's final charge against Cochise.
During the Grand March at the dance, the actors perform the first round (couples) leading with the dance-favored right foot. When they double up to 4s and then 8s, they are leading with the military-favored left foot.
As the regiment is heading out, we see Philadelphia standing to the far left of Mrs. Collingwood and Mrs. O'Rourke. Soon after, she is on the far right, which indicates she moved. However, right after the close-up of her standing on the far right, the next immediate long-shot has her standing on the far left again.
When Lt. Michael O'Rourke and Philadelphia goes riding, they stop for while with a high and peaked boulder on their left-hand side. In the next shot, when they renew to ride, seen away from the other side, the boulder has changed to a large and low one.
During the dance in the Sergeants Mess, when the dancers are coming round in front of the camera in fours, one of the dancers curtsies, but none of the other women do this. They had already done so in a previous shot and obviously the woman must have been confused.
When Capt Yorke reports to Col Thursday at the NCO's dance, he says that Cochise and his people have crossed the Rio Bravo back to American soil. This movie takes place in Arizona, there is no river separating Arizona from Mexico. The Rio Bravo (Rio Grande in US) separates only Texas and no other states from Mexico.
The canyon that Captain York and Sergeant Beaufort ride into to meet with Cochise is supposed to be in Mexico. It is the same canyon that Col. Thursday and his command ride to defeat in the United States.