Some rifles are set very light - perhaps too light - ie: setting the trigger at 1/2, then cocking the piece can trip the sear which fires the gun - THAT I've seen happen - and probably happened to the fellow Roger was talking about. I've seen people set the trigger before mounting the rifle and the gun went off - pulled the wrong trigger back. Most people who set before mounting, don't look at the trigger before setting it- get the wrong one and pow!
As far as I'm concerned, setting before coming to full cock, is just asking for an accidental discharge, just as going to full cock before closing the frizzen - That can cause an accidental discharge as well - when unset - the vent gas usually burns the hand of the shooter, which is a good reminder not to do that. Hope the muzzle was pointed down range.
This light trigger setting is also period correct, BTW - some rifles would fire when set if the muzzle was raised too high - that's documented. I've no idea where, look it up - seems to me the Swiss taget rifles were mentioned.
As far as no fingers inside the trigger guard until you are ready to fire, that's not only common sense, but standard safe gun handling - in my 40some years of shooting & handling firearms. Seems to me, I had to answer that finger/trigger guard question in my safe gun handling test to get my first hunting licence - age 15 - it was an easy question to answer then, too.
Because you've seen no one 'called' on it, means nothing.