Airmile hackers figured out that they could buy dollar coins (free shipping as you know!) from the US Mint, using credit cards that gave them airmiles for purchases. Then they took the dollar coins straight to the bank and deposited them, paying off the credit card bills before any interest was incurred. The only cost was gas for the bank-runs. The Mint has tried to put a stop to it.

Another exploit for British Airways desire to woo continental passengers is for Brits to register a PO Box in France and use that as their address for the BA frequent flier club. People who live in continental Europe level up to perks much faster than people in live in Britain. For the cost of an annual PO Box, you can save thousands of pounds on getting into the lounge and getting free massages, meals, etc.