A lot depends on what is interpreted as a "safety" - on a pistol with a frame mounted safety, the firearm will not fire with the safety engaged (assuming of course that it is mechanically functional). However, much has been written about those who may not train regularly fumbling with or forgetting a safety - or whether it is safe in the up or in the down position - under duress. So an acceptable solution has become a monstrous trigger pull, likely in combination with a grip safety and/or trigger safety. One scenario is that, with gun in hand (depressing the grip safety), finger properly OFF the trigger, the holstering action allows some shirt material or something to depress the trigger, and since all conditions are met, the weapon fires. Obviously, holster design can be important. As far as forgetting to use a decocker, well you can forget to engage a safety, or forget to lower a hammer, etc, etc someone will find a way to defeat just about anything.