Visible crew/equipment: When Salma Hayek is tossing the bullet she has just removed from Antonio Bandaras's shoulder, you can see the reflection of Robert Rodriguez holding the film camera on the metal dish.
Continuity mistake: During the shootout at the Tarasco bar scene, there is a part where El Mariachi is shooting bad guys from the front of the bar. He switches from sitting to lying almost face down throughout the scene. (00:32:00)
Continuity mistake: Antonio Banderas throws his guitar case across the roof, then jumps after it and opens it. From the reverse angle the fake guitar inside is still in place, we then switch to it from from the front and it's instantly raised, with the guns visible instead.
Answer: It is called "Pass The Hatchet", and it is written by Ray Theriot, Roger Leon Jr. & Earl Stanley Oropeza. It was performed by Roger & The Gypsies. If you buy the soundtrack CD, it is on there, too.
Deezul