From an excerpt from Steven Levitt and Stephen Dubner's new book Think Like a Freak:

Van Halen's live show boasted a colossal stage, booming audio and spectacular lighting. All this required a great deal of structural support, electrical power and the like. Thus the 53-page rider, which gave point-by-point instructions to ensure that no one got killed by a collapsing stage or a short-circuiting light tower. But how could Van Halen be sure that the local promoter in each city had read the whole thing and done everything properly?
Cue the brown M&M's. As Roth tells it, he would immediately go backstage to check out the bowl of M&M's. If he saw brown ones, he knew the promoter hadn't read the rider carefully—and that "we had to do a serious line check" to make sure that the more important details hadn't been botched either.
And so it was that David Lee Roth and King Solomon both engaged in a fruitful bit of game theory—which, narrowly defined, is the art of beating your opponent by anticipating his next move.
Both men faced a similar problem: How to sift the guilty from the innocent when no one is stepping forward to profess their guilt? A person who is lying or cheating will often respond to an incentive differently than an honest person. Wouldn't it be nice if this fact could be exploited to ferret out the bad guys?
We believe it can—by tricking the guilty parties into unwittingly revealing their guilt through their own behavior. What should this trick be called? In honor of King Solomon, we'll name it as if it is a lost proverb: Teach Your Garden to Weed Itself.

I'd read the brown Van Halen brown M&M story before in Atul Gawande's wonderful book The Checklist Manifesto, but this excerpt contains one story I hadn't seen before, about the medieval ritual of adjudicating by some horrific ordeal.