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.
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.
Other mistake: When Blondie and Tuco are talking about telling each other the location of the money, a car drives by in the background. In the shot where both Blondie and Tuco are visible, watch the top right in the trees in between the pillars supporting the bridge.
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. (00:14:30)
Continuity mistake: When Blondie is saved by the mortar shell, and it shows the empty noose, it is completely different than the noose that Blondie had around his neck. The loop that the noose went through to hold it to the beam has changed.
Plot hole: After Blondie has "teamed up" with Angel Eyes, he hears a gunshot and says something like "Every gun sings its own tune", indicating that he's recognized the shot as being from Tuco's gun. But how did Tuco get his gun back? It certainly would have been confiscated when he entered the prison camp, and the chance that the guard on the train has that exact gun is extremely unlikely - he would most likely have had an officially issued sidearm.
Suggested correction: It's the type of gun Tuco likes that makes that particular sound.
Revealing mistake: When Blondie is to be hanged by Tuco, and saved by an extremely lucky hit by a Union mortar, you can see that the wall that will be hit by the grenade has been blown 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. (00:47:10)
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. (00:17:50 - 00:18:50)
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.
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.
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 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.
Other mistake: Director Sergio Leone uses one of his trademark filming techniques throughout this film, where anything outside the camera frame doesn't exist. Several times the characters react to situations and other characters when they come into frame that they logically would've seen much sooner. For example, at one point Blondie and Tuco are walking and stumble across an army camp. Even though in reality they would have been walking toward it for a long time and would have been aware of it, they react with surprise the moment it appears on screen.
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). (01:59:15)
Continuity mistake: When Tuco tosses the rope to Blondie to hang himself with, the rope is all tangled up with no knot showing. In the scene break that cuts back to Blondie, the rope has changed to a nice neat coil with the hangman's knot showing.
Revealing mistake: 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.
Factual error: When Angel Eyes arrives at the Confederate Army Hospital, the Confederate flag that is flying is not the correct flag; it is the flag that people in the 20th century and later think of as the Confederate flag. The only rectangular flag with the blue St Andrew's cross on red was the Confederate navy flag and the battle flag of the Army of Tennessee, which was the standardized battle flag for the Western Armies (but not that far west) after January 1864, so didn't exist in the New Mexico campaign, nor at the time the story takes place.
Visible crew/equipment: When Angel Eyes has finished paying the legless man for information, the legless man hops into the saloon. Notice he has two shadows, revealing the presence of stage lighting.
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.
Visible crew/equipment: When Angel Eyes sits down at the dinner table in his first scene, there is a small unlit lantern hanging on the wall near his head, and unexplainable light sources reflected on it.
