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.
Factual error: The guns Tuco and Blondie use appear to be 1851 Colt Navy because they have octagonal barrels and the loading ram below the barrel. 2 problems are that 1) the '51 was only made in .36 calibre. When shown from the business end the muzzles appear to be much larger. Colt '61 / '60 came in .44 caliber, but they had round barrels. 2) More importantly they are using brass cartridges to load. Conversions for those pistols to load cartridges did not exist till after the war and these were cap 'n' ball only then.
Other mistake: When Blondie is cleaning his pistol he only loaded 3 bullets, but as Tuco's bandits enter the door Blondie shoots off 4 rounds.
Continuity mistake: When Tuco pulls the corporal from train they fall down a steep grade. When train runs over the corporal, they're on flat ground. The corporal would be too heavy for Tuco to carry far enough for the ground to flatten out. Watch the ground when the train approaches before the jump.
Factual error: While cleaning his pistol Eastwood is using a plastic bore brush, not available in that era. (00:00:45 - 00:50:00)
Factual error: Regarding the train scenes when Tuco is taken then escapes from Mario Brega, there were no trains / railways in New Mexico in any of the years of the US Civil War, let alone 1862 when Selby's short-lived and daft campaign was limited to. I believe the RRs were starting to move into Texas then, but not NM until the 1870s. Also, the whole geography of the film is wrong if the gold was intercepted but 'escaped' with the three people in the wagon near Glorietta Pass in early 1862. The sequence of events means all involved had to go up and down the Rio Bravo constantly - eg. the 'jornado del muerto' is hundreds of miles from Glorietta. This is too complicated to summarise here, but check the actual battles and timelines.
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.