When the Baxters and the Rojos are shooting at each other at the cemetery, Ramón Rojos runs over the tops of the rocks to get a better shot. As the Baxters fire at him, there is one bullet that hits the rocks under Ramón that had no sound of a rifle being fired nor any ricochet sound.
In the final shoot out with Ramon, Joe loads only one bullet in his revolver, then quickly spins the cylinder. This would have given him only one chance in 6 that the cartridge would be under the hammer when he pulled the trigger. One experienced gunfighter would have caused the cylinder to click only four times. With practice this can be easily and quickly accomplished by dragging the cylinder from the base of the middle finger to the fingertip (with small changes to allow for differences in the size of the hand).
When the large barrel rolls down the ramp, it rolls on two large rails. Right afterwards Joe crawls down the ramp, and no rails are visible. The large rails re-appear when his captors come into the room right after he exits.
The action before, during, and after the graveyard fight alternates between taking place during the day and night. The Rojos leave in pitch darkness, cut together with scenes of the Baxters riding out in cloudy daylight. The fight takes place during the day while Joe sneaks into the Rojo house at night. When the Rojos return home, it's night again.
When Joe meets Don Miguel, the suit of armor has holes in it. In the plaza, when the Rojos are partying, the suit of armor has no holes in it until Ramon shoots it. It would seem the Joe/Don Miguel scene was shot after the plaza party scene.
When Joe and the Shop Keeper see the soldiers leaving town in the morning, the Shop Keeper is getting dressed and leaves his pants unbuttoned and shirt askew from center. Then the next scene in the same room his pants are completely button and his shirt opening is centered and buttoned up.
While Chico carries Joe back to his room after the party, the camera shows Joe has no facial hair. Also, when Joe enters his room after the massacre, Rubio is a different actor; Ramón's mustache is parted.
In the hostages exchange scene, when Marisol is still at the other end of the main street, Ramón tells her quietly, "Go to him, Marisol". This must be an error in editing as her child only runs into the street later.
When the two men are shooting at the child's feet to scare him, they shoot more than 12 rounds, even though each of them is carrying but 1 revolver holding 6 rounds, meaning that they cannot possibly have between them any more than 12 rounds loaded.
In one scene showing the hills beyond the town, the ruins of Tabernas Castle can be seen on the hilltop, showing that it was filmed in Andalucia, Spain. The castle was used by Fernando and Isabella during their siege of Almeria in the 1480s.
When the Rojo gang ambush the Mexican army unit the gun Ramon uses to kill all the troops is a Mitrailleuse volley gun. Each barrel had to be laboriously loaded by hand before all barrels were fired together in a single volley. However, the film shows the volley gun being used as a form of machine gun. The only machine gun around at the time was the hand-cranked Gatling gun which the soundtrack also seems to depict.
When Ramon shoots the escaping soldiers, by the river, in their backs, he does so with a Gatling Gun. Yet, when the "shot" soldiers cry out and fall to the ground, no blood or even small holes can be seen in their clothes. A Gatling Gun would rip them (the soldiers, as well as their clothes) to pieces.
Joe tests his bullet-proof vest using a revolver. In the gunfight, however, Ramon is using a Winchester rifle and those bullets are much more powerful than a revolver's. When Joe takes off the vest during the fight we see the bullets have made the same depth of imprint as when he tested it with his revolver. They should have been deeper.
In the scene where the Mexican cavalry are ambushed at the river, the machine gun used by the 'US Cavalry' is unloaded at all times - it is a belt fed weapon, yet has no belt of rounds in it - it presumably has caps in the end to produce the muzzle smoke. Furthermore a hand cranked Gatling Gun would be more appropriate to the time line the film is set around than the slightly more modern air cooled belt fed weapon used.
When the gang open the safe and reveal the fortune inside, the safe is practically full of Confederate States banknotes. The movie is set around ten years after the end of the American Civil War, which would make the CSA banknotes worthless.
The scene where the Baxter's and Rojo's exchange prisoners, when Marisol begins to move forward on her horse, there is a window in the BG that reveals there is no back wall to the building and the distant landscape can be clearly seen through it.
After Joe sets off the dynamite, he's standing in the street where the strong wind is blowing to the right of the scene. Yet we see the buildings behind him, where the dynamite smoke is blowing to the left of the screen.
During the ambush of the 60-some soldiers, there are no bullets hitting either bodies, the sand, or in the background, and not one drop of blood is spotted, although the soldiers are acting like they are hit, there is not one bullet hole among them.
When Joe escapes the wine cellar after his brutal beating, after he sets off the rolling barrel down the ramp to take out Chico and the other baddie, a special effects wire can be seen on the right side of the barrel half way down the ramp that presumably sets of the exploding barrel effect when it hits the closed door.
After the shootout at the graveyard (between the Baxters and Rojos) Joe is carrying an unconscious Marisol and hides near the corner of the building when the Rojo gang returns to the compound. However, it appears that Joe and Marisol are clearly visible to the Rojo gang.
The 1977 prologue for TV featuring Harry Dean Stanton has a heavily swathed body double for Clint Eastwood shot largely from behind. Two tight close-ups of Eastwood's face with his trademark squint have plainly been taken from other footage, as he's squinting in bright sunlight and Stanton is talking to Eastwood indoors.
In the final fight at the end of the movie - he shoots 6 shots in total at the Rojo gang - then fires a 7th shot to free the man hanging on the rope without reloading. The revolver he is using can only fire 6-shots and then needs to be reloaded.