Fistul of Dynamite:
Steigers shot is shot by someone hiding behind the rock. Barrel of rifle can be seen moving after the shot.
Shadows of the stagecoach drivers comes from an additional lightsource, not the sun, compare with the shadows in the background.
During the robbery of the Mesa Verde bank, just after Juan (Rod Steiger) shoots open the first vault containing the prisoners. There is a frontal shot of Juan's gang and you can see an extra dressed in a soldier's outfit rushing down the stairs towards them. He is not carrying a rifle or any gun, more importantly, he disappears by the next shot. Juan and his men definitely don't shoot him, and he's nowhere to be found.
In the cave scene when Juan's boys have been killed among the tens of others, the small kid you can see with open eyes: look carefully at his stomach, you can clearly see him breathing, to make it even more funny, the kid can bee seen breathing even more clearly in the scene where Mallory feels remorse for Juan.
Thanks to everyone who spotted bloopers and sent them.