The film is about western outlaw Billy the Kid, who was actually right-handed.
The "Englishman" describes his origins as from Ayrshire, a county in south-west Scotland. In that case he would be a Scot, and not English. John Henry Tunstall was actually from London, the capital city of England.
Billy and the others are at a lake. There's a reflection of the moon in the water. One of them shouts "the moon" and they start shooting the reflection of the moon.
When the camera zooms out, Billy and his pals are facing the camera, so the moon was at their back all the time. Hence there couldn't have been a reflection of the moon. Since they weren't shooting the Moon, it must have been something else, possibly a reflection of a UFO.
In one scene at Garrett's wedding, the song "La adelita" is heard in the background. This song was composed during the Mexican Revolution, several decades later.
After Joe Grant draws on Billy, Billy draws, too, and tosses two of Grant's bullets on the floor to show how many he had surreptitiously removed from Grant's gun. The bullets land on a rug-covered brick floor, but give off a sound of having hit a bare wooden floor.
At the 15 minute mark, when Billy is supposedly alone up the pass where Tunstall has just been gunned down, one can see the head and hat of an extra, presumably on horseback, in the bottom left corner of the screen.
Paul Newman, at 33, was obviously miscast as Billy the Kid who was murdered when he was only 21 years old.
Billy is supposedly a fluent in Spanish, but when he arrives at Pat Garrett's pre-wedding party, he addresses his host, Miguel Saval, with whom he is long acquainted, as "mi-GWELL". In Spanish, Miguel is pronounced with a hard "G", and without a "W" sound.