Latest i heard was that it was a .40 semi-auto that supposedly had a round in the chamber when it went off... "d'oh" is right.
I'm not a "gun person" but it seems to me to be foolish to have such a weapon "cocked" when it really doesn't take long to unsafe it and chamber a round and fire it, if needed... an accident waiting to happen.
I can see keeping the mag in; fumbling with that in an emergency could be a critical time-waster. But there's no need to have it set up for a "quick draw", I think.
It depends on the weapon. Oddly enough, the oldest design, is still the best. John Browning's design in the form of the tried and true, Colt 1911A1 is designed to be carried in 'condition one' (loaded magazine, round in the chamber, hammer cocked). There are two safety systems. A very positive hammer lock that can be quickly dis-engaged by the firing hand's thumb... and a grip safety that won't allow the firing-pin sear to release unless fully depressed by the web of the shooting hand.
Stored or holstered in condition one, the 1911A1 can be brought to readiness by a flick of the thumb, .. and even then, if not firmly in the shooter's hand, the firing pin cannot reach the chambered round. It takes series of deliberate actions in order for a bullet to exit the barrel.