Revealing mistake: After Manco beats up Cavanaugh and places Cavanaugh's back up against the bar, while Manco faces him, three of Cavanaugh's henchmen appear at the entrance of the bar demanding that Cavanaugh be released. Manco turns around and kills all of them with his pistol, while Cavanaugh falls to the floor crawling toward his gun. Manco is walking toward the entrance of the saloon with his back to Cavanaugh. Manco draws his gun and kills Cavanaugh without looking at him. If you look closely at the tip end of the pistol, you can see that the only thing that Manco would have killed was the saloon floor. (00:17:00)
Revealing mistake: While Lee Van Cleef is browsing the news-journal, we first see a front page, followed by 6 identical consequent pairs of pages and then again the same front page from before. Also, the text doesn't seem to be text at all. (00:48:10)
Revealing mistake: During the "dream sequence", Indio shoots the young man several times, yet there's neither blood nor bullet holes on his shirt, front or back.
Revealing mistake: About an hour and a half into the movie Clint Eastwood forces a telegraph guy to send a fake message saying the bank is being robbed, in a nearby town - as a ruse. In the next scene, when the posse rides down a dirt road, you can see tire tracks as they are all coming down a hill and going around a bend.
