Factual error: During the scene where Navajas is throwing knives at El Mariachi and then the Bucho gang in the limo, he slides down in front of the car. The license plate simply says "Mexico" under the serial. Actual Mexican plates of the time period in the movie use the official state abbreviation followed by MEX to identify the state of issue. For example, JAL MEX for Jalisco, which is where the JWF685 serial seen on the plate would have been issued.
Continuity mistake: When the knife throwing assassin is after El Mariachi, he throws the first knife at his left arm and the second one at his right hand. Then we see El Mariachi going around the corner, with most of his back out of view, as the assassin throws the third knife at him. A few moments later, we see El Mariachi taking the knife out of his back - it's right next to his shoulder, and it was impossible for the assassin to hit it from where he was.
Continuity mistake: Near the end of the film, Antonio Banderas calls his friends and holds the phone in his left hand. The camera cuts to Salma Hayek and then cuts back to Antonio where he is seen to have the phone in his right hand.
Continuity mistake: When the fight breaks out in the bar during the opening song with El Mariachi and his bandmates, he motions for them to join him to break up the fight. After he breaks it up his bandmates reappear on stage like they never left.
Continuity mistake: After Antonio Banderas saves the boy from the truck, they are walking beside a building. The guitar the boy is carrying flips over with every shot change. First, the strings are facing the boy's leg then the guitar back is facing the boy's leg. It changes 4 or 5 times.
Other mistake: In the bar fight/shoot out Antonio Banderas shoots a fan onto one of the gunmen and we see the man fall down face first into the camera with the fan on the back of his head, Antonio walks around the bar and guy is suddenly laying on his back and turned completely around with fan on his chest.
Audio problem: When the kid shows El Mariachi how he plays, you can hear the part that he's not playing because it's a constant sound on one string and he's stroking all the strings at the same time.
Continuity mistake: When the little boy is giving his fathers guitar to the drug runner it has no strings, but when the drug runner gives it to Antonio Banderas it has a string.
Revealing mistake: When Antonio Banderas runs over the guy at the end of the big shoot near the end of the movie, the guy getting run over has been replaced with a dummy. You can tell because of how easily it moves and how much it twists. You can also slow the film down to see that the dummy's plastic hand is visible.
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.
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.
Audio problem: When the bad guys in the car are shooting at Danny Trejo, the kid with the broken leg kills him with six shots we see and hear, but the slide on his gun locks back after 5.
Continuity mistake: When the man sitting next to Tarantino is shot, in one shot there's no blood on Tarantino's face and neck, but in the next shot there is.
Revealing mistake: In the fight scene in the Tarasco bar, one of the bad guys jumps on El Mariachi, but he stops him with his feet and pushes him off and shoots him as he is flying through the air. As the bad guy is shot, you can see the tape holding the squibs on his shirt.
Continuity mistake: In the fighting scene with Antonio Banderas and his two guitar friends, before everybody starts shooting we see the little boy walking around strumming his guitar. In that first shot the strap is crossing the back of his neck. But in the very next shot of the boy, that rope is hanging down almost on his knees.
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.
Visible crew/equipment: When Banderas and Catalina are on the rooftop, and Bucho shows up and Banderas doesn't shoot. When they start going to the next rooftop, you can see a shadow of a microphone going up the wall.
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.
Answer: Tito Larriva as Tavo says (in reference to Steve Buscemi) that "This guy smells like shit." The name of the song being played is "Manifold de Amour" by Latin Playboys.