Points For Purchases
A local restaurant chain near me ran a special where every purchase gains you points, and every 100 points you get a $10 discount on your next purchase. It turns out to do this, they would just credit $10 to your account, which was usable when you used their rewards card.
So I purchased a $100 gift card, and got $110 in value. Then I used the $100 gift card to buy another $100 gift card, and now had $120.
I stopped there once I confirmed the loophole, and the staff was laughing pretty hard about it. The rules quickly changed.
Story credit: Reddit / Schwerlin