Continuity mistake: When the Amigos are deciding to "be the three amigos for real", Dusty is the last one to agree. Before approaching the other two, he grabs for his gun and it discharges, hitting the ground with the shot. Glass is then heard shattering in the distance. At this point, Dusty had not decided to be a "real Amigo", so his gun would either be loaded with blanks or unloaded, not loaded with live rounds yet.
Revealing mistake: During their first confrontation with El Guapo, the Amigos start riding around and firing their guns in the air. Look at the difference in speed between the wider shots (with stunt doubles) and the closer ones, which show the actors themselves. Not only are they suddenly moving much slower in these closer shots, but it so very obvious that they are on mechanical horses by the movement.David Mercier
Continuity mistake: When the 3 stop to drink from their canteens, Ned gets sand and some mud on his face. When he looks at Dusty, there's now a darker mud stain on his right cheek that wasn't there before. And in the close up, the stain is gone but he has a lot more sand on his face, including all of his nose.Bishop73