Continuity mistake: The number of knives on the knife thrower's chest holder change throughout the scene.
Audio problem: Why didn't we hear the 18-wheeler come rolling down the street when the little boy was going to get El Mariachi to come with him to get his guitar? He crosses the street and then the sound appears and the truck appears.
Continuity mistake: When Bucho shoots at the limousine glass when it first arrives, the shot on the window changes around a couple of times.
Continuity mistake: During the rooftop shootout Hayek has a red and a black shoe on. At one stage you see both shoes in her hands then there's a shot of Banderas and then its back to Hayek and she's taking the red shoe off again.
Other mistake: How does Banderas shoot the bad guys falling back off the building? It's impossible from the angle he's at.
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