Continuity mistake: At the start when Mary gets off the buckboard she has a food basket with a couple of things but no cloth/bread, but upon entering the house the basket has a red and white check cloth and bread sticking out.
Continuity mistake: When the Elders are at the ranch waiting for John to show up, Tom and Bud are talking and Tom has a deck of cards in his hands. When he is talking to Bud you can see that the top card is the 10 of hearts. The shot goes to Bud and when it comes back right away to Tom the top card is now the 3 of hearts.
Continuity mistake: When the sheriff enters the bar to break up a confrontation between John Elder and Curley, he sends John Elder out of the bar. As Curley starts to follow Elder out, the sheriff tells him to sit down and finish his card game. Elder and Curley were standing facing each other for quite a while before the sheriff came. How would the sheriff know that Curley had been sitting there playing cards? (01:03:00)
Continuity mistake: When the Elder brothers decide to go see their old ranch and ride out of town, Tom has a noticeably scruffy beard. When they get to the ranch and have a run-in with the deputy, he's clean shaven. They ride back into town with the deputy and he has the beard again.
Continuity mistake: When the Elders are being escorted from the jail to be taken to Laredo, as they walk out it's a dull day and they don't cast any shadows. When the camera angle changes to the other side of the road, the sun is bright and they all have strong shadows.
Revealing mistake: When John Elder is at his parent's grave site and the sheriff comes up behind him, John Wayne has two guns to make it appear that he draws very fast. Both guns, the one in his hand and the one in his holster are briefly visible as he turns to aim the gun at the sheriff. Viewed on VHS.
Other mistake: Moving them to Laredo early in the morning, except it's noon by the wagon shadows.