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)
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.
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.
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.
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)