Revealing mistake: As Jango fires the machine gun, the ammunition chain hanging from it is loaded on both sides, and it doesn't move. Also the belt has different sizes of cartridges in it. (00:45:30)
Revealing mistake: During Cole and Mississippi's night patrol, a bunch of horsemen start making a fuss in the street. When Mississippi dives under their horses, there is an interruption between when he dives, and when he (supposedly) lies in the horses' path. Evidently, this scene was made by joining two cuts.
Revealing mistake: When Blondie is to be hanged by Tuco, and saved by an extremely lucky hit by a Union mortar, you can see that the wall that will be hit by the grenade has been blown to pieces at least once before; the bricks have no mortar between them, and there is a large circular scorched area on the brick wall. (00:47:10)
Revealing mistake: In the last scene, when Chris is leaving, it shows several shots of the peaceful village. Each time they show the village, in the middle of the square is what looks like two boys fighting. One punches the other and he falls down and gets up and punches back and he falls down. They do that about three times.
Revealing mistake: At the very end, Belle is replaced on the horse by a very obvious dummy wearing a dress, just as the Rumpo Kid jumps onto the saddle.