Revealing mistake: When Antonio Banderas jumps into the Cadillac to escape from the bikers, there is a wide shot of him driving the car while being chased for a few seconds where you can very obviously tell he is not driving the car. It is a stunt double, and a very skinny one at that. In fact it almost looks like a woman.
Revealing mistake: Watch carefully when El Mariachi says "Then I guess I have no choice but to kill you all" to Cucuy, and he fires a shot. You can see a black outline on the shirt of the guy's shirt who's hit right before sparks shoot out of the place of impact. This happens on other occasions too.