riddle about two criminals with their crimes tattooed on their face; must guess their own crime?
So I have vague memories of a riddle/short story/fable/morality tale, possibly relayed in a TV show or movie, about two criminals hanging in cages who have had their foreheads tattooed with their crime -- and they don't know what their crime was. They're not able to see their own tattoo, only the other man's. Criminal A sees that Criminal B has something utterly terrible like RAPE + MURDER, and tells him so -- but when it's Criminal B's turn, he reacts with utter shock/terror/etc so that you're left wondering what Criminal A did that could be worse.
Either you're supposed to figure it out yourself, or there was an answer provided -- I can't recall anything. Is this story ringing any bells?
EDIT: It was a story within the play "The Pillowman" by Martin McDonagh! Hooray for this community. :D