Created by Barbara Mikkelson, one-half of the Urban Legends Reference Pages (www.snopes.com) team. Her Some Idiot Rule states that no matter what the situation, or the gravity of said situation, some idiot, somewhere, will do something idiotic.
Her most recent example is the Starbucks employee who charged rescue workers $130 for three cases of water.
In a similar vein, see also Sod's Law, Murphy's Law and the 50/50/90 rule.