+1 to what's being said above. A single action revolver in the cocked position with no sort of internal safety or protection from dropping? Then I could see it happening, but it seems like the sort of weapon that isn't fit for carry. That's why the two common and popular types of carry weapons are double-action revolvers (very heavy trigger pull, near impossible accident discharge and dead reliable) and semi-auto pistols with safeties of all kinds (grip safety, internal safeties, firing pin blocks, etc.) that ensure the weapon only discharges when the trigger is pulled. Or even DA/SA semi-auto pistols that are a combination of both! I've owned handguns that would go off if dropped. But I'd never carry them.
The reality is, those aren't unknown factors. I know something could snag the holster, I could drop the weapon (pretty hard for it to fall out of a good retention holster but that could happen too). Because we know these things we have trigger guards on our holsters, and handguns that have been engineered with these issues in mind.