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.
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.
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. (00:54:10)
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.