Revealing 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)
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)
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)
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. (01:00:18)
Revealing mistake: 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. (00:25:08)
Revealing mistake: When Tuco is trying to avoid Blondie firing at him with the cannon, there is a shot of him running while an explosion occurs next to him, followed by another shot of him leaping to the ground and rolling over. In both of these shots, Tuco is replaced with a stunt double. (02:36:06)
Revealing mistake: When Tuco climbs back onto the train after he uses it to cut off his chains, the film appears to have been sped up, as his bodily movements appear unnaturally fast. (01:56:40)
Revealing mistake: When the cannon fires at the hotel, in the shot of the room that Tuco and Blondie are in, it explodes; Tuco appears to have been replaced with a stunt double. The man can also be seen bracing for the explosion. (00:49:11)
Revealing mistake: When Angel Eyes shoots Stevens, he does so by firing mostly forwards, plus a few degrees upwards. This causes the soup in the bowl on the table to blast upwards, and the bowl itself breaks apart. However, the gun doesn't appear to be aligned with the bowl or the soup in a way that would cause it to be affected by the bullet being fired, especially seeing as how it was aimed at Stevens' head. (00:14:20)
Revealing mistake: When Tuco first enters Sad Hill Cemetery, you can see the dual tire track ruts left by the dump truck that brought in the sand to make the grave mounds. (02:36:35)
Revealing mistake: When Tuco is about to shoot Blondie in the desert, and the Army wagon is seen approaching, the driver can be seen sitting far back in the rear of the coach wearing a hat. (01:06:33)
Chosen answer: Tuco knew he had a better chance at survival (and the gold) by playing along with Blondie and keeping his balance on the cross, hoping Blondie does not miss the rope when he shoots. This final, deadly game was preferable to being shot outright by Blondie and dying a poor man.
Scott215
Tuco had no idea that Blondie's intention was to shoot the rope and let him live.