To me, the amount of inaccuracies in the film are so great that it wouldn't surprise me if a rocket showed up too.
Cars are supposed to show up on at least three occasions in GBU, though I don't remember them. There's a car driving along a road in the background of
The Searchers too. It stops to watch the cavalry crossing a river! It's a very meta experience when that happens, because you suddenly stop suspending disbelief.
That's also the same with firearms with me, or German uniforms in war movies.
The thing is that Leone knew what he was doing for the most part, but he didn't care. So he would mix and match the percussion and cartridge weapons from scene to scene, passing them off as being identical, and show them in close up as well.
He was so precise with his film making that he would sometimes spend hours, if not days, until he was happy with a shot. He would have clothes weathered so much that they looked on the point of falling apart, as if to highlight the reality that was lacking in Hollywood films, particularly from the 1930s to 1950s (think Marty McFly's BTTF III pink 1950s cowboy outfit!)
Yet, when it came to one of the biggest parts of the mythology of the west - firearms - he played fast and loose.
This close up illustrates the problem:
View attachment 539403
The metal cartridges are superfluous, and couldn't be loaded into this Remington.
The percussion caps are actually fitted here too.
IMFDB records that one of the few percussion weapons fired on set was Angel Eyes' Remington Cattleman's Carbine.
Once Upon a Time in the West likely had its anachronisms too, probably with rifles when it came to weaponry, but they weren't so glaring.
To differentiate between the two, GBU is more a cartoon, and OUATITW more a serious work. This seems how Leone approached the films. As a case in point, Clint's character was superhumanly fast with a gun in the Dollars trilogy. However, in Harmonica's first shootout, while he kills the three men waiting for him, he gets shot in the shoulder by Woody Strode's character.
I think it was Alex Cox who said that OUATITW ought to be in everyone's top ten westerns.
The way I see GBU is that it's an alternative history of the west, which is really the essence of most films anyway.