During the shoot-out after the bank robbery, the robber behind Eddie is holding a shotgun, then a pistol, and then a shotgun again.
At the climax, the lamp hanging from the ceiling of the jail swings with a period of about 2 seconds which mean that the rope from which it hangs was 1m long (which makes for an unusually tall ceiling). Shortly after it swings with a period of 1 second which would mean the rope was shortened to 0.25m.
When Ben is talking to Molly Cain on the widow's front porch, her mailbox can be seen on the fence in front of her house. The movie is set in 1870, but the postal service didn't start delivering to rural mailboxes until 1903, and that style of mailbox wasn't invented until 1915.
When the dead Marshal Hiram Cain is brought back into town strapped onto a horse, he is clearly breathing when his widow is standing by him.
Just as the closing The End title appears, the dead Eddie Campbell lies on the scaffold where he's been shot, but his arm is clearly moving.