As Morgan shoots at Sartana, his hilltop perch looks substantially different in long shots than it does in close-ups.
As the stagecoach chase progresses, the stagecoach and riders get closer to the trees and the curve in the road behind them, rather than farther away.
On the top of a barrel at 52:57 a brown cup is visible to the left of a lantern. A few seconds later and throughout the remainder of the scene the brown cup is located to the right of the lantern.
At the end of the film, large clouds of dust and hay billow in the street, yet the leaves on the tree in the foreground are perfectly still. The dust and hay are obviously being blown by large fans off-camera.
At about 22:22 into the movie, one of the gamblers is smoking a plastic tipped cigar. They did not have plastic tipped cigars in the Old West.