Revealing mistake: After Joe (Clint Eastwood) shoots dead the five men in the "small house" there is a close-up shot of one of the dead men lying across the table and his eyes are still open. Joe then pushes him off the table and as his body falls the dead man closes his eyes. (01:02:38)
Revealing mistake: When Ramón and his men massacre a detachment of Mexican soldiers by ambushing them with a Gatling gun, it's easy to tell that the soldiers are just pretending to be hit by bullets. There isn't a single bullet hole or drop of blood on any of the soldiers also in the wide shots not one horse is hit in the crossfire nor has the carriage been damaged by gunfire. (00:25:43)
Revealing mistake: When Clint Eastwood ambushes the two guards by crushing them with a large barrel you can see two extended rails have been added to either side of the ramp, presumably to help with the stunt. (01:11:20)
Revealing mistake: During the prisoner exchange scene, when Marisol is told to move forward on her horse, if you look at the building behind her you can see it's a partially built set. Through the buildings window you can see the desert. (00:53:05)
Revealing mistake: About 40 minutes into the film, there is a shot of guys on horses going one direction. Before the shot cuts, a road sign pole is visible. This indicates the camera crew are on a dirt road.
Answer: The look in Ramon's eyes.