The Navy did demand that S&W revolvers add a hammer block safety device to their revolvers though. The Colt da revolvers had such a safety device in them for many years but not S&W. After a few unintended discharges with S&W revolvers dropped on steel decks the Navy objected and S&W did add the hammer block. They have had them to this day.
The S&W Revolvers of pre WW2 had a hammer block, but it was in the form of a side swinging L shaped piece with the longer leg forming an integral flat spring.
I have a .32 Handejector with this earlier hammer block, it was made in the 1920's.
The change was to a more positive block that was moved by the action of the trigger or the rebounding hammer slide rather than relying on spring pressure.
Both systems use the same channel cut into the side plate, so the older revolvers could be upgraded at little expense.
The side swung block could be disabled by congealed fouling or rust in the channel, if the spring became weak enough. A hard knock that dented the side plate could also put it out of action.
Aside from apocyphal stories like that of Black Jack having an AD (with Patton being in the room when it happened rather than it happening to him) and the rather mysterious incident I mentioned, the only other incidents I've heard of involving a 1911 going off in a military holster involved paratroopers with a chambered round going off when the chute opened. The sudden deceleration shock of a parachute might do it.
Any autoloader can go off when dropped if the breechface is badly dinged up near the pin hole, or a bit of hard grit got in there while racking the slide.
A firing pin jammed in the forwards position by grit could also do it.
A missing or very weak retraction spring would make mishaps more likely.
Milspec primers were hard enough that it would require some serious impact, but I have heard of autoloaders going off if the primer cup of commercial ammo was to soft.
I just remembered something.
A Tanker holster I had used with my 1911 had a very tight strap that passed over the grip (some modern replicas have straps that pass over the rear of the slide so it will be between hammer and slide if carried cocked and locked) and that strap depressed the grip safety.
On a couple of occasions the thumb safety was disenaged when I put the gun in that holster. The very tight fit, due to shrinkage in storage was probably the culprit. When oiled and broken in those problems went away.