The car seen driving down the dirt road is visible during the conversation from the passenger on the train. This occurs before Mortimer lowers the Bible. The car is seen out the window moving from lower right to upper left.
Playing cards of the Old West did not have modern numerals on each corner. The number cards were represented by the number of spades, hearts, clubs, or diamonds in the center, six of diamonds had six diamonds in the center. Also, the cards in the movie were modern machine cut plastic coated paper, period cards would have been larger, and of plain pressed paper or wax coated.
Mortimer smokes a meerschaum pipe throughout the movie, which is historically accurate; however, the pipe stem is obviously Lucite, a late 20th century plastic. An 1860's pipe stem would be carved of ivory, antler, or bone.
When Manko climbs onto the roof of the hut to break through the roof to steal the loot, his face is normal. When he climbs inside and there is a close-up of him reacting to seeing that someone else has already beaten him to it, as this point Eastwood has darkened grease paint on his face but when he climbs down to join Mortimer, his face is once again clean of any grease paint.
After Mortimer exits the train, he goes up to the ticket office and spots a wanted poster for $1000 which the wanted baddie had added 2 more zeros to the bounty. When Mortimer tracks the man down in a hotel room and slips the same poster under the door, the extra 2 zero's are now missing from the poster.
After Colonel Mortimer pulls the emergency cord aboard the train, the locomotive crew is seen applying the brakes. There's a shot from inside the locomotive cab backwards across the tender, and there are no cars connected to the engine.
When Mortimer unrolls the wanted poster before sliding under the door, it is obvious that the poster has a curled-up appearance. Once it's under the door inside the wanted man's room it is suddenly completely flat.
When Mortimer slides the wanted poster under the door and knocks, the bad guy fires four times through the door. When the chase continues into the street, the bad guy fires six more times. However, he is carrying only one gun, which he did not have time to reload.
When Monco springs Perez from jail, the sun is only just barely starting to rise on the horizon. Yet the ground the two of them race across is perfectly well lit, which would only be possible in the day, or with artificial stage lighting.
At the end of the movie when the Colonel shoots El Indio, El Indio's left hand that's holding the watch is by his left side. When the Colonel goes over and steps on his hand to get the watch, his arm is stretched out.
In the end of the movie, just before the duel, we see El Indio reloading his gun. He uses all the cartridges in the lower left side of this ammo belt. However, when he goes out to the street, 4 cartridges appear again.
When Mortimer confronts the Indio gang for the first time and lights his pipe from Wild's cigar, there is a blond woman behind who can be seen watching at them at first, then sitting on a table, then staring at them again.
When Manco rides into Agua Caliente, several women run down the street into their houses. They are followed shortly by another woman, but the door to her "house" doesn't open. She simply tries to hide from the camera behind a short wall.
When Monco shoots three of Indio's men outside of Santa Cruz, he takes his dark brown horse. However, in all his scenes in Santa Cruz, he is seen with a completely different horse. After he leaves Santa Cruz, his horse is dark brown again.
At the end when Monco is loading bodies into the wagon and drives away, a tailgate is clearly visible hanging down on the back of the wagon. A minute later after he gets the saddlebags of cash, as he pulls away the tailgate is gone.
Near the film's conclusion as the two main characters exchange farewells in separate frames, Mortimer's close-ups and long shots show him against a setting sun. However, Monco's scenes, as the short shadows around him demonstrate, is in light that is much closer to noon.
When Indio is lying on the ground after being shot dead by Col. Mortimer, Indio's left hand (still holding the gold watch and chain) is turned palm down, with his left thumb pointing towards his body. In the next shot where Col. Mortimer retrieves the watch from Indio, Indio's left hand is now turned with his palm up and his left thumb is pointed away from his body.
When Col. Mortimer pours the acid into the drilled holes in the safe, he tilts the dropper more than 90 degrees each time. If it were real acid this would've caused the rubber bulb at the end to be quickly eaten away, giving him chemical burns in the process.