The cast and crew stayed in Las Vegas while working on this project. Actor Woody Strode wrote in his memoirs that he and Lee Marvin pulled several pranks, including shooting an arrow at the famous smiling cowboy neon sign damaging it briefly.
During the filming of the scene where Maria attempts to escape through a canyon wired with dynamite, Claudia Cardinale's stunt double was badly injured during the explosion. Cardinale, who had never ridden a horse before, performed the stunt herself for the final cut, and escaped uninjured.
The "French 75" mentioned in the dialog refers to the 75mm field gun artillery piece, Model 1897, issued by France to America's National Guard during World War I and also used by the Mexican army.