Given the time of year, when I read about cheating and the honor system, my first thought is about taxes, specifically, paying state sales tax on items bought online and shipped from out of state.
Most states don't have an easy way of catching sales tax cheaters, so it usually comes down to the honor system: When you file your state income tax return, you are supposed to add up all your "mail order" receipts for the year and pay sales tax on them.
How many HNers actually do this, I wonder? This seems like an instance where there is an honor system, but everyone cheats anyway.