When Raul is crouching behind the cars in the final gunfight he runs out of ammo for one of his pistols (the "bedazzled" one) and he throws it away. Later, he is seen using the same pistol when he is shot.
(at around 6 mins) When Ferrel is hiding behind the rock, the background is a two-part screen and was not lined up straight. However this movie has a lot of deliberate mistakes. A very obvious painted background, not lined up straight, is just one of them.
The goof item below may give away important plot points.
Near the end of the movie, when Armando gets thrown the rifle to shoot Onza, you see the camera behind the bullet as it travels to it destination. What is shown however is a bullet with the casing still attached. Normally when firing a bullet, the shell stays behind (and/or is ejected from the weapon) and only the bullet travels downrange.