Factual error: After the birth of the baby, James Stewart and "Doc" Witherspoon are discussing the course of the war. "Doc" relates that he has always lived in Virginia and that his son died at Gettysburg at Little Round Top. No Virginia regiments fought at Little Round Top - only Alabama, Texas, Georgia, and Arkansas regiments.
Character mistake: The 4 brothers have just rode into town and are securing their horses. On the flag pole in the background the Texas flag is upside down. Note: The correct way is white flies on top and red on bottom.
James Kennedy
Deliberate mistake: Near the beginning, Col. Mortimer takes down a wanted poster and goes after the man. The man he kills looks nothing like the man pictured on the poster.
Suggested correction: There is a similar situation in the UK produced western movie "Carry On Cowboy" (released 1965). Johnny Finger/The Rumpo Kid (Sid James) shoots the sheriff of a US town. The sheriff is related to legendary sharpshooter Annie Oakley (Angela Douglas) and she swears revenge. Finding that Johnny Finger is staying in a saloon, Annie Oakley sets up a trap on the staircase. Another person (an uncredited extra) inadvertently walks up the staircase, so she shoots him instead. As he falls down the stairs she calls out "I'm terribly sorry I thought you were somebody else." Perhaps Col Mortimer has made a similar mistake. Being less polite than Annie Oakley he does not have the good manners to apologise.
Suggested correction: Is that a mistake? (Either on the part of the people who made the film, or assuming this was real life, on the part of Colonel Mortimer?) They're a pretty bloodthirsty lot in this film, and nobody seems to really care who gets shot. Maybe he figures he can get a bounty for that person anyway.
Rob Halliday
Audio problem: When the band is marching with the Massingale rally one can hear flutes and many band instruments, but the band consists of only a few brass instruments.
Character mistake: Ryan's final diary entry says the US Cavalry fought against "French irregulars." The French lancers were actually regular troops.
Necrothesp
Revealing mistake: In the scene at the end of the movie where Lee Marvin is running interference during the escape by shooting in the air, horses pulling a beer wagon get startled and run into the street where the wagon turns over.At the beginning of the shot, when the horses begin to bolt, you can see the stuntman driving the wagon. He is sitting low behind the wooden front of the wagon (which has holes cut in it so he can see out).