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".
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 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 newspaper tacked to the wall of Grant's railroad car (visible as he is looking at the files of the men he is hiring) proclaims that Pancho Villa and Emiliano Zapata have entered Mexico City. The Villistas and Zapatistas occupied the city on the 6th December of 1914. According to Grant (Ralph Bellamy), Fardan (Lee Marvin) had served with the rebels through 1915.
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.