I have a little to add on this thread but I am only familiar with one type of pistol, the H&K USP 40 in Double Action Only variant. I am personally aware of 2 negligent discharges (ND) that happened when holstering this weapon; in both cases the trigger was pulled, either by the operators trapped finger or the safety thumb-break strap of the holster.

The procedure to prevent this from happening is to place the thumb of your gun hand on top of the hammer of the weapon as it is seated in the holster to prevent it from rising if the trigger is being pulled. If any movement of the hammer is felt the holstering process is stopped immediately, the situation is inspected/cleared, the weapon is withdrawn safely, unloaded and both the weapon and holster inspected for obstructions or defects.

I can in no way say that this is what caused the ND that happened in the airliner cockpit (or even if they use this type of pistol).

The above holstering procedure is just how I was trained and it has worked well for me.

Mike