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 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. (00:47:45)
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. (00:48:25 - 00:49:33)
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)
Continuity mistake: When Tuco stands back up after falling through the floor at the hotel, there is dust on his poncho. Then it cuts to a closer shot where he is shown looking up at the floor above, and there appears to be less dust on his right shoulder. (00:49:27)
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. (00:50:15)
Continuity mistake: When Angel Eyes is talking with the Confederate soldier at the hospital camp, whilst the soldier asks him, "And you, have you ever heard of someone named Camby?", he picks up a handful of wood pieces so that he can throw them into the fire. It then cuts to a closer shot of the two, and there are fewer pieces of wood in the man's hands. (00:52:42)
Continuity mistake: Whilst Angel Eyes is speaking with the soldier at the Confederate camp, when the soldier is saying, "I just don't know what you drifters could be thinking," Angel Eyes has his back hunched forwards. In the next shot, he is standing upright. (00:52:55)
Continuity mistake: When Tuco shoots the water bottle after it lands on the ground, water begins leaking out of the bullet holes. The water is shown in front of the bottle when it leaks into the sand, but a minute later, when Blondie and Tuco continue moving, the water is shown to the left of the bottle. (00:58:00 - 00:59:15)
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)
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. (01:02:35 - 01:29:28)
Continuity mistake: When Blondie grabs Tuco's shoe, take note of the plants to his left. There are a few to the very left of the shot. But two shots later, after it is revealed Tuco is washing his feet, there are more plants to the left of him. (01:03:04)
Continuity mistake: When Blondie attempts to have a drink of water from the bowl, Tuco flips it over. It then cuts to a shot of Blondie, and the bowl is shown flipping over a second time. The bowl is also closer to the ground the second time it is shown. (01:04:03)
Continuity mistake: Blondie collapses from exhaustion and rolls down a sand hill. When he reaches the bottom of the hill, there is no sand on his neck or his shoulder. When the shot cuts, there suddenly is sand on his neck and right shoulder. (01:05:30)
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)
Continuity mistake: When Tuco stops the run away wagon in the desert, Blondie's pistol is not in his waistband. As he walks back to the wagon, it is. (01:07:00)
Factual error: Extended version. After finding a dying Bill Carson, Tuco arrives at an outpost looking for a hospital. It's night, or at least dark outside - he is told the hospital is 18 miles away. In the next scene he arrives at the hospital at nearly high noon. A horse canters and gallops (full speed) between 4-30 mph. Basically the hospital was an hour away. (01:14:50)
Continuity mistake: A dirty Union cavalry officer hits himself to clean off the dust that made him appeared Confederate. His first two strikes barely shake the dust on his uniform, yet, after a cutaway and two off-screen strikes, his arm is almost completely free of dust. (01:31:05)
Character mistake: At the confederate prison camp after they are captured a soldier comes up to the sergeant (Van Cleef) and informs him that the "Captain" wants to see him right away. In the next shot with Van Cleef where he is talking to the commanding officer you will see that he is not a captain but a major. (01:35:00 - 01:35:29)
Continuity mistake: When Angel Eyes has finished looking at the small case owned by Bill Carson, he closes it. He has two fingers on top of the case, and the other two fingers, as well as his thumb, on the bottom of the case. When the shot cuts, his thumb is on the top of the case and the two fingers on top of the case are closer together. (01:37:35)
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.