Goofs

When the caravan stops overnight and Cooper guards the stagecoach with the gold in it, as Lancaster approaches the stagecoach we can see Cooper looking inside and then he closes the door and starts to turn toward Lancaster. However, the scene immediately cuts to Cooper still looking inside the stagecoach and turns around in surprise to find Lancaster right behind him.

When Emperor Maximillian is checking out the Winchester, his chest sash has a large golden Mexican eagle which repeatedly disappears and reappears between shots.

Around 01:17:15, Ballard is between Pittsburgh and another man. Two shots later, he's alone on the right.

During the final battle the character Abilene (played by James Seay) simply disappears. He is last seen leading some Mexican revolutionaries. Most likely a cut or deleted scene.

In a scene of fighting with the girl, her silver earring is visible on the ground BUT when seen again, she is wearing both earrings.

The Gatling Gun used by Emperor Maximilian's troop with a drum magazine wasn't available until October 1884. In fact, since the Gatling Gun wasn't invented until well into the American Civil War 1860s (1862 in fact) it's questionable that Maximilian would have one in the base case.

The name of the Mexican city was originally two words, but now it is officially one word: Veracruz. However, at the time of the movie (in the late 1860s) both Vera Cruz and Veracruz were in common use.

When the wagon train is ambushed by the Juarezistas in a town, they make a break for it, fanning out and riding over an expansive field. The wagon tracks from previous takes are all visible in the grass.

The French lancers are using what appear to be disguised Model 1904 U.S. Army MaClellan saddles.

At the beginning of the final battle the mercenaries number eight members. Five of them (Joe, Ben, Pittsburgh, Ballard, Donnegan) storm the city while the remaining three (Tex, Abilene and an uncredited member) stay back providing covering fire. Four of the five are killed except Ben but the film ends with no conclusion on what happened to the remaining three. It is also inconclusive if Ben collected the $100,000 payment for assisting in taking the city.

It's possible the remaining three were killed off-camera however all were safe from returning fire behind cover and at a far distance as their Winchesters were needed to simulate artillery. So did they survive and if so did they and Ben split the $100,000 four ways or did Ben get the entire amount? The film ends without saying.

During the rifle marksmanship display at Maximillians ball, they use Pittsburghs rifle, a winchester 1894 which did not exist in 1866. Also the Americans carry colt SAA pistols not available until 1873.

Burt Lancaster's character is seen smoking a filtered cigarette which weren't available until 1931.

Joe's saddle gun, a "new, Winchester repeating rifle", is an 1873 model; since the film is set "just after the American Civil War", to be correct, the gun should be a Henry "Yellowboy", circa 1866. The earlier weapon had no wooden forearm stock and a brass receiver (this is incorrect, it was the 1860 Henry repeater had no forearm stock)

This film is set in 1866. The handguns used are 1872 Colt single-action army revolvers. The rifles are even more of an anachronism - they appear to be 1894 Winchesters.

When Burt Lancaster looks at the gold in the carriage, he triggers some sort of devise that automatically raises the heavy gold boxes and opens the two covering flaps. It looks to be electrically operated. It then reverses itself when the gold is lowered back into its compartment later in the scene. This seems to be a well advanced technology for the time and overly complex for simply carrying gold coins in a hidden compartment.

Joe Erin says "Always did like kids. How about you, General?" to General Ramírez, who replies "Certainly" without opening his mouth.

The very first scene was clearly shot in the town of Tepoztlan (State of Morelos), as apparently was a significant portion of the movie. Why the group would then choose to go north through Teotihuacan (past the Pyramids) in order to reach Veracruz (which is located on the East Cost) is beyond me, as it wouldn't make any sense at all. That's the equivalent of going from Des Moines to Minneapolis in order to reach Chicago.