Continuity mistake: When Blondie abandons Tuco in the desert, watch the rope around Tuco's neck. It changes between shots, sometimes over his left shoulder, and sometimes hanging down in front of him. It is not due to the camera angle. (00:28:27)
Revealing mistake: When the stagecoach driver gets hit with an arrow, it springs up from below into a horizontal position. (00:33:55)