Revealing mistake: When El Mariachi's one friend first fires a rocket at the bad guy's limousine in the climactic battle, you can see that the rocket itself never explodes, despite the side of the car blowing up. The rocket simply lands next to the car, and then remains there in plain sight whilst the side of the car blows up, revealing that it is just a prop replica.
Revealing mistake: When Antonio Banderas shoots the guy with the broken ankle at the beginning of the shoot-out on the roof near the end of the movie, he falls off. Slow-mo his fall and you'll see the air mat he's suppose to land on, it's a huge evenly raised lump in the sand about a foot high and surrounded by white rocks. He even sinks into it when he lands.