Audio problem: When we see the can-can dancers perform on-stage, the music is played by a full band including brass and drums, despite the fact that there's only a pianist on-stage. (00:22:10)
Madstunts
Other mistake: In the very last scene of the movie, and just before Blondie shoots the rope that is holding Tuco on the grave's cross. You can see a car moving in the background, screen right of Tuco's head.
Continuity mistake: In the final scene at the sheriff's office, Mitchum has his feet up on the table top, and the boot soles are clean. But after Wayne walks out, there is suddenly a spot on a boot sole. (This is visible in the video)
Deliberate mistake: When Raza is chasing the "empty" train, as it goes over the trestle just before he stops it, you can see the engineer in the train's engine.
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: 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)