Revealing mistake: In the jail scene, Gene Hackman is reading from a book. When he turns the book too far towards the camera you catch a glimpse of his lines on brighter white paper attached inside the book.
Revealing mistake: When El Mariachi is trying to escape Moco's men by sliding down a wire onto the bus, he first throws the ball-and-chain-weapon over the wire. But then when he slides down, you can see a pulley device that he is using, not the chain. (00:26:45)
Revealing mistake: In the scene when Crow Horse and Levoi are running from Coutelle and the "GOONS", they go off-road toward the stronghold. You can see in the grass the tire tracks from all the other takes. Especially as there is no reason any vehicle would leave the road at that place otherwise.