Revealing mistake: The first time we see the artillery blow up a group of Confederate soldiers, the guys wear pretty distinctive hats; the one at the center has a red band on it, the one to his right a straw hat, etc. Ten seconds later, the artillery blows up another group of soldiers...who are the exact same group from before. It's just another take. (00:00:30)
Continuity mistake: Mr. Thad Benedict is in front of the Langdon residence asking if the property is for sale. As the widow answers, the position of his hand on the coach turns around, and the natural light changes as well. (00:10:00)
Continuity mistake: When the Union soldier is reading the note tipping them off about the caravan about to cross the river, he holds the paper with his left hand placed in an entirely different spot even with the cut between shots happening literally mid sentence. (00:31:40)
Continuity mistake: At the end of the movie, when John Wayne followed by the harmonica player is talking to Marian McCargo, her scarf goes from left to right or right to left depending on the angle of the shot. (01:56:00)
Visible crew/equipment: In the last scene, when they are riding alongside a river, shortly after the harmonica player changes songs, there's a closeup of John Wayne and appearing from the right on the other side of the river, a pickup truck appears, and then they quickly cut to different characters. (01:56:55)
Factual error: When the Confederates enter the plaza to be greeted by the Mexican authorities, the Stars and Bars are raised below the Mexican Flag. However, during Maximilian's reign, the Mexican Flag was completely different from the flag shown.
Factual error: The Civil War has just ended, and John Wayne is herding horses to Mexico, yet he carries a Colt Peacemaker, not manufactured until 1873.
Answer: Because as far as they were concerned the battle was not over, the South would rise again.