The shiny rock
The people began to believe that to ward off thieves, they should go to sleep at night with a shiny rock in their pocket. It would multiply the thief's punishment tenfold.
Naturally this didn't work, but one day a particularly slow and superstitious thief lifted up the quilt to check for the shiny rock. He instead saw a naked wife and a partly clothed, very angry husband - who beat him to death with the shiny rock he happened to have on his night stand.
Rumor quickly spread of the shiny rock's power. Soon thieves everywhere were terrified. You just couldn't tell who had a shiny rock in their pocket, and soon everyone did have a shiny rock in their pocket, so you couldn't tell who didn't. It didn't make sense. But nobody wanted to risk it, just in case.
As for the husband, that wasn't his wife, and he beat the thief to death so that no one would find out he was an adulterer. So it goes.
