Elmer Keith wrote about it in Gunnotes with specific mention of two different packers who got shot down through the legs while saddling or unsaddling horses. The standard practice was to hang the near stirrup on the pommel when saddling/unsaddling; in each case a heavy stirrup had dropped on the hammer of a holstered sixgun, firing it. IIRC Keith also recounted hearing the 'hammer down on an empty chamber' admonition from old gunmen when he was a kid, which would be the early 1900's.
I recall an incident from my own childhood. One of my Dad's acquaintances had a Colt SA, which he kept loaded full-up as a house gun. One day he had it out of the dresser drawer, for some reason, and promptly dropped it. Murphy was in charge of gravity that day and arranged for the Colt to land on its hammer. This sent a big roundnose slug ambling off from floor level at a slightly upward angle, through two interior (plaster) walls and out the kitchen window. Mrs. Deadeye was cooking her little heart out in there, had just walked across the bullet's path on her way to the table, and turned around just in time to see it knock plaster all over her floor and bust out her window. This would have been about 1963 and I'm certain it was not a high point on the Domestic Bliss Timeline for that particular family.
Incidentally, Elmer Keith saw the New Model Ruger, with its transfer bar mechanism, as a huge improvement in single action revolvers. Like many of us, he noted that the new mechanism didn't help the trigger pull any; and he trusted that dedicated gun cranks would find a way around that problem.