The Gravelings. An entire race of spiky, fanged gremlins whose sole purpose is to set up accidents which inevitably end in the random, senseless and usually gruesome death of some unwitting soul? Brrrr. The regular gravelings were never explained and appear to just be agents of chaotic death with no particular hatred toward humanity... just amused by the "accidents" they make happen. But when when reapers actually committed the murder, they become a far more deadly one. Because they weren't supposed to die, you get little balls of rage that want nothing but revenge against the living. And they're invisible and silent to almost everyone!
We never actually see it happen, but Rube's description of how a soul goes bad if you leave it in the body when it should have died brings very unpleasant images to mind.
George fails to reap a man's soul before his death occurs. He's trapped inside his body while his autopsy occurs. It's exactly as disturbing as it sounds.