Lee Marvin was drunk throughout a lot of the filming, and was in fact so drunk during a scene atop a giant rock that assistant director Tom Shaw intervened out of fear that Burt Lancaster would "take Lee Marvin by the ass and throw him off that mountain".
After Burt Lancaster ambushes Jack Palance's men and shoots Jack Palance. Palance shouts out for "Francisco". The following shot of a dead Francisco also shows a horse having a bowel movement in the background.
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.
Plans for a remake were announced in 2000 but as of 2011 haven't been followed through. Among the people attached were screenwriter Bruce Feirstein and director John Woo.
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.
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.
The shooting was plagued by many complications including rain, snow, sleet, the blazing sun, intense desert heat and even a flash flood.