OK, hate to name names but the holster was a five-year-old Kramer made out of horsehide. I perform around 20-30 drawstokes daily, and fire about 400 rounds a week if that qualifies as practice.
I worked outside for several hours in about 100+ degree weather getting my range ready for a 3-day shotgun class. Between the tight fit of an IWB and my sweating, the leather became damp enough so that the front serrations on my Kimber, the area around the tigger guard and the ejection port molded to the pistol. After I came inside and it cooled off, the "fit" was pretty permanent. You're a holster maker and I work with leather myself quite a bit (including making two saddles) so you understand about hand-boning a holster. I could not have hand-boned it more perfectly to the fit of the pistol if I had worked on it for a couple of days.
One of the other folks I referred to was Louis Awerbuck. IIRC he was wearing a Summer Special when the same thing happened to him.
To tell the truth, I love leather and hate Kydex for handguns, and this may not be a factor in an area with low humidity/temps. But if this has happened three times that I'm aware of, it will happen again--probably at the worst probable moment. I'm stickin' to Kydex for IWB.
Denny