Revealing mistake: When Mongo wakes up and breaks free of the chains, in the shot where he breaks free, the chains suddenly shift position and some even disappear. This is because the camera was stopped and the chains were replaced with fake break always before the camera was restarted. Another way to see this is because the water that dripping down Mongo's face suddenly stops.
Revealing mistake: When the Waco Kid supposedly steals the king from the sheriff as they are playing chess, the real king falls out of the sheriff's hand as he pretends to grab it. It still lies there throughout the rest of the scene.