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.
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.
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 amber, ivory, antler, or bone.
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.
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.
When searching old newspapers to determine who is competing with him in El Paso, Colonel Mortimer discovers a front page of the El Paso Tribune from June 15, 1872 with a story about Monco that includes a photograph. The first photograph printed in a newspaper would not occur until March, 1880..
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 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 Nino is stabbed in the back by Groggy, his dead body falls in front of the door with enough room for Groggy to walk passed his left side to get to Indio, and in the close up shot, his head is clearly turned to the left. But in the next shot after Groggy is in front of Nino's body, his head is facing the other way, and his body has closed the gap that Groggy has just walked through.
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 Manco and Mortimer are loading their guns ready for the final showdown, they clearly have a few bruises on their faces from their beating. But once the shootout begins, they are never shown with any bruises again.
Manco leaves White Rocks with a different horse that he arrives with. The one he is first seen with is the same dark brown one he later rides into El Paso on. But the one he rides off on as he leaves White Rocks, is a much lighter brown one,
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.
In the wide shot of Indio's men pulling the safe into the wagon from the bank, Indio is on the left hand side of the frame riding off on his horse. But in the next close up shot, he is stationary on his horse, pointing his pistol at the bank.
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.
At the beginning of the film, the Guy Calloway wanted poster has two hand drawn circles added to the printed $1,000 to make it look like $100,000. A few minutes later when the same poster is slid under the hotel room door the two hand drawn circles are gone.
After Monco picks up the reward for Cavanagh and tells the two townsfolk "You people need a new sheriff", he jumps on his horse. However, his right foot noticeably misses the stirrup. A second later he tries again, and misses again. Later he turns his horse around and after he walks past the camera, you can notice him slightly bending forward and checking, making sure to get it right this time.
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.
At the very beginning, Mortimer (Lee van Cleef) pulls the communication cord in the passenger car, to stop the train at Tucumcari. However, when the train draws to a stop, Mortimer emerges from the horse box, which has no access from the passenger car.
During the "Hat Duel" both Manco and Mortimer shoot each other's hat numerous times. However, when they are talking in Mortimer's hotel room afterwards, you can see the hats sitting on a table and only one bullet hole is visible in each of their hats.
Mortimer (Lee van Cleef) forces the train into an emergency stop at Tucumcari so he can get off with his horse. The conductor, after confronting him, walks up the ramp into the horse car, yells to the engineer to pull away, and closes the sliding door. The conductor is now stuck in the horse car since it has no doors on its ends facilitating exit to other train cars.
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.
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.
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 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.