Billy shoots several holes in the town sign near the beginning of the episode, but the holes were present in a previous establishing shot.
In the week leading up to the OK Corral battle, Wyatt Earp introduces himself as the local Marshal. In fact, Wyatt Earp's older brother Virgil was the Marshal, with younger brother Morgan as deputy. Wyatt, a saloon keeper, was deputized as an emergency measure on the walk to the battle site.
Doc Holliday is still practising dentistry, even though the episode is set more than four years after he last publicly offered his services as a dentist.
A camera can be seen in the mirror behind the bar.
Doc Holliday's common law wife Kate speaks with an exaggerated American accent, but probably really spoke with a Hungarian accent.
When giving Steven and Dodo the keys to their hotel rooms, the barman uses the British term for one floor above ground level, calling it the "first floor." The story is set in America, where the first floor refers to the story at ground level. He should have told them their rooms were on the second floor.
After seeing the town sign for Tombstone, Arizona, the Doctor tells his companions that they are in the mid-west. Tombstone is several hundred miles from the mid-west.
Doc Holliday was only 30 years old when the Gunfight at the OK Corral occurred. Consequently, it would be difficult to confuse Hartnell's Doctor with a 30-year-old Doc Holliday.