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: Having a tough-looking, silent henchman/henchwoman is a pretty common trope in action movies. She's basically there to look tough and mysterious. I wouldn't read into it more than that, especially given how minor the character is. (Minor enough that I had to Google her, because I couldn't even remember her, and I've easily seen this movie a dozen times over the years).
TedStixon