When Chet is painting the gun, smudges on the car door can be seen. In the shots where the car is stopped, the smudges are gone, also the painted paper is turned.
Later in the end of the movie when the van drives next to them, the paint smudges are also not on the door.
During the pen gun scene, while Chango (Michael Pena) has The Major (Fred Ward) at gunpoint, as the camera alternates between frontal and rear shots of Chango you can see his trigger finger switches between being on the trigger when seen from behind but outside the trigger guard and resting along the frame when seen from the front.
The bank guard's gun could not have fired as depicted. A blow to the hammer of a modern double action revolver (like the one carried by the guard) cannot cause a discharge because the hammer block safety remains automatically engaged until the trigger is pulled.
When Nick and Chet are being chased in the Datsun, Nick shifts gears, indicating he is driving a manual transmission car. However, after he shifts and is performing his 180-stunt, a close-up shot shows his feet and there is no clutch. In a manual transmission car, the pedals are close together, and the brake pedal is smaller due to the clutch next to it.