Factual error: Towards the end when the cavalry uncovers the American flag, it has 50 stars.
Continuity mistake: When Bugs is performing his shooting stunt in the saloon, you can see the entrance behind Sam has nothing between it. When Sam tells Bugs he's missed, we cut to Bugs who says "Wait!" and then return to Sam, and this time there are doors between the entrance.
Continuity mistake: Sam Boone walks Ben Brigade to the horses holding his rifle, and they have a bit of a chat by and about the captive Billy. During this scene, the handkerchief at Randolph Scott's neck waves like mad in the wind over his shoulder under one camera angle, and it is still and in front of the chest in the other. (00:09:30)