Now for my original question.Is there a case of anyone getting himself shot while trying to undo a loop?
I have not heard of anyone shooting himself while fiddling with the hammer loop or thong. The loop goes
over the hammer spur, and hammer need not be cocked at any point while the revolver is in the holster. You pull the loop over the hammer spur with your thumb and forefinger, then pull the free end of the thong to pull the loop down snug. Friction keeps everything in place. When you want to deploy the revolver, pull up on the loop with your thumb and forefinger and push it off the hammer spur. Either operation literally takes about one second when you learn how. If you think you're going to need to draw your revolver (or "skin yer hogleg," in the patois), you remove the loop well in advance. The hammer loop is a very simple contrivance, practically foolproof, and it works very well.
I have heard of a thong punched for a hammer-mounted firing pin which goes
under the hammer to retain the gun in the holster. Maybe that's what you were thinking of... You would need to at least partially cock the hammer to get the thong under it, or to remove the thong for drawing the gun. To me, this seems like a bad design, an accident looking for a chance to happen. I could easily see a nervous or careless person shooting himself in the leg with that type of arrangement. I would never use one.
I have heard of cowboys being shot in the leg while saddling their horses, though. If you have a live round in the chamber under the hammer in an unmentionable sixgun with a hammer-mounted firing pin, even a light blow to the back of the hammer could result in an accidental discharge. The old-time cowboys would drop the near-side stirrup over the saddlehorn to get it out of the way while tightening the cinch. If the horse spooked or jumped, the stirrup might come off the saddle horn and flop down, and whack the hammer of the holstered sixgun. With a live round in the chamber under the hammer, the gun could easily "go off" and shoot the cowboy in the leg.
Some years ago, a friend of mine was at a local range on a weekday morning. He was the only one there, and he decided to practice his fast draw with live ammo in his revolver. Long story short... He put a round in his leg. Please don't make any smart aleck responses. He was a good man, a straight shooter in every sense of the word, but he showed a momentary lapse in judgement. The wound was relatively minor, but he was on Coumadin for A-fib, and bled heavily. He did require surgery to repair the wound, and they had to stop the Coumadin for the procedure. Post-operatively, his cardiac issues became problematic due to the break in his Coumadin therapy, and resulted in his death just a few days later.
So, please, no fast draw with loaded revolvers! Ever! If you feel the need to try fast draw with your cap-and-ball revolver, load caps only! Or just dry fire it and say "BANG!" Fast draw is pretty much a Hollywood phenomenon. I've read that on those rare occasions when shootouts did occur back in the day, people faced off with guns already in hand.
Be safe!
Notchy Bob