 There are a few references from the graphic novels of "Sin City",  which Rodriguez filmed after "Once Upon A Time In Mexico". First,  when Sands shoots Ajedrez in the stomach after getting kissed by  her, its a identical picture from the graphic novel "A Dame To Kill  For" that is number 2 in the Sin City maxi series. Earlier in the  movie, when Barillo makes Sands blind, there is almost an exact  picture from the Sin City book "Hell And Back". Other similarities  are the scene with Mariachi and the priest in the confessional booth  (as seen in the book "The Hard Goodbye") and the subplot involving  facial reconstruction (as seen in "A Dame to Kill For").