Money that you find where you least expect it; or, money you receive that is not deserved or earned. Such money is often believed to be cursed. In such a circumstance, the "Law of Found Money" applies: Spend the money as quickly as possible, or misfortune will occur.
I found a $20 bill in the Wal-Mart parking lot the other day. I spent it right then and there. It's the law of found money, you know.
Many lottery winners experience bad luck that is associated with the enormous winnings; I believe this is the law of found money at work.