"The officer thought he had removed all of the rounds from the cylinder of his .38-caliber weapon when he pointed it at a wall and began to explain "trigger pull pressure." As he pulled the trigger, a round that had been left in the chamber discharged into the wall."
A little hint for those who handle wheel guns...count the rounds after you drop them out of the cylinder. Look at the cylinder to make sure no rounds are left, next count the rounds again. If the weapon leaves your hand, open the cylinder and check it the next time you pick the weapon up.
I can't imagine anyone, except perhaps a drunk in the dark, not noticing a cartridge left in the cylinder.