Somewhat similar to the 'office supply fraud': http://www.snopes.com/crime/fraud/supplies.asp

Or, for that matter, debt collection agencies sending notices for expired debt, and having small-print at the bottom explaining that you are under no obligation to repay this, but if you respond and/or make a payment, they'll be able to go after you for larger amounts. (I have one on my desk. I might frame it for when I need to look at something and have a chuckle.)