The doctor at one point mentions "living in a never never land," an obvious reference to Peter Pan. However, the play did not see its first performance in London until 1904, and the novel was not published until 1911. There is no way anyone living on the 19th-century American frontier could know of Peter Pan's home.
When Quirt pushes between the men at the bar, the bartender pours him a drink and then leaves. The owner then arrives asking for some calm. As Quirt leaves the bar, the bar owner can be heard from behind the Baker brothers still imploring them not to fight, but it is the bartender who can be seen.
A dance hall girl pushes a spectator down onto a gambling table which collapses under him, with the only damage being to the legs of the table. In the next shot, the table is still collapsed but there is a large section of the edge broken off.
When walking down the street for the final showdown, the sun starts off to Quirt's right casting shadows to the left of screen, then a close shot shows shadows which could only come from a near-overhead sun, and then at the saloon the sun is coming from Quirt's left casting shadows to the right.
When Penny wheels the wagon around to pick up Quirt at the beginning of the movie, you can see an extra set of reins coming out of a small "window" that is visible below the wagon seat where the actual wagon driver (the stunt driver) is controlling the team of horses.
In the next to the last scene in the movie where the Marshal (Harry Carey) shoots Hondo and Laredo, he kills them with two rapid shots before they can shoot. In the reverse it shows the Marshal cocking his lever action rifle. It would be impossible to fire two shots so rapidly if you had to lever the rifle between shots.