When the U.S. government issued the Sacajawea dollar coin, they wanted to get them into circulation as quickly as possible so they’d catch on (Narrator: they didn’t), so they had a thing where you could order them and have them shipped to you for free.

People realized you could pay for the coins using one of those credit cards that gives you frequent flier miles as a reward. So you order $1000 of coins, put it on the card, get the coins, deposit them into your bank, pay off the credit card. You’ve just gotten 100% free frequent flier miles.