Other mistake: In the very last scene of the movie, and just before Blondie shoots the rope that is holding Tuco on the grave's cross. You can see a car moving in the background, screen right of Tuco's head.
Revealing mistake: After the explosion blows a hole in the wall and Tuco joins the procession of people flowing down the street, when the shot switches to face the characters you can see that the 'woman' in a blue dress in the wagon is actually a man. (Time is for the extended version of the film).
Factual error: When Clint Eastwood blows up the bridge, he lights only one fuse but all of the explosives detonate simultaneously, despite being attached to numerous, separate pilings.
Factual error: When Tuco and his associates come to kill him, Blondie stops cleaning his pistol and quickly loads it with brass cartridges, which were not available for pistols until 1873.
Continuity mistake: When Blondie and Tuco are planting dynamite to blow up the bridge, they are standing in the water. During the following blast, they are hiding away next to the river and from then on, they are totally dry again.
Continuity mistake: Towards the beginning of the movie, Angel Eyes rides up to a house where he has to get information from a man inside and then kill him. After Angel Eyes kills him at the kitchen table, his son runs down some steps behind Angel Eyes in the background with a rifle in his hands. Angel Eyes pulls out his revolver, turns around quickly, and shoots him dead. Take note that the pistol is not pointing directly at the boy as he is firing it.
Continuity mistake: When Tuco is riding into the three-man bounty hunter ambush, one of the men fires a shot, presumably killing his horse as Tuco is not shot, and if the horse was only startled it would rear back and not tumble forward. The then dead horse is missing from the rest of the scene.
Revealing mistake: When Blondie is to be hanged by Tuco, and saved by a extremely lucky hit by a Union mortar, you can see that the wall that will be hit by the grenade has been has been blow to pieces at least once before; the bricks have no mortar between them, and there is a large circular scorched area on the brick wall.
Plot hole: In the scene when Tuco has ordered Blondie to stand on a stool and put the hangman's noose around his neck, he orders him to tighten the noose. Just as Tuco aims his pistol to shoot out the leg of the stool, an explosion causes the entire floor to collapse. Tuco drops to a lower level but logically Blondie should have dropped also, thus hanging himself. Instead he mysteriously disappears and lives on for the rest of the movie.
Other mistake: Blondie suffers from severe sun burn wounds in the face, after the capture and dragging through the desert. The wounds look perfectly realistic, but disappear or change several times. The burns are completely gone shortly after leaving the monastery. Such burns would leave scars and traces for at least weeks.
Continuity mistake: When Blondie abandons Tuco in the desert, watch the rope around Tuco's neck. It changes between shots, sometimes over his left shoulder, and sometimes hanging down in front of him. It is not due to the camera angle.
Continuity mistake: When Tuco is riding his horse, he encounters three bounty hunters - one fires a shot making the horse misstep, and fall, bringing Tuco down to the ground. One of the bounty hunters ask Tuco, "Hey amigo, you know that you have a face beautiful enough to be worth $2,000.00?" He has a big blade of grass between his lips as he backs away from Blondie (Eastwood), however, there was no blade of grass between his lips when Blondie and he were close to one another. As he backed away, he had a rifle in his right hand and a wanted poster in his left, and he knew that Blondie had a gun pointed at him through his overcoat, so he was not able to put a blade of grass between his lips at any point.
Factual error: When Tuco is in the gunshop messing around with the guns, one gun has the cylinder and barrel moving back and forth by manipulating the trigger guard-lever. This is a Belgian Galand & Somerville revolver, a gun that did not exist until 1868. This film takes place during the US Civil War, which ended in 1865.
Visible crew/equipment: When Tuco makes Blondie march through the desert, there is a shot at the very beginning and Blondie looks over his shoulder, feeling the strain of the desert. Then the camera focuses on Tuco and in this shot, you can see the tracks in front of his horse from a previous take.
Revealing mistake: When Tuco kills the fat man he is chained to whilst in the desert, in many shots this supposedly dead man can be seen breathing. Especially noticeable when he is on the tracks.
Factual error: Just before Angel Eyes confronts the girl in her room about the whereabouts of Bill Carson, we see her walking in the hall outside her room, and lighting this hallway is a very modern day lightbulb.