We got a $100 gift card to a restaurant chain and used $40 one night for food and left a cash tip. They give you a credit card looking receipt and we just left it without doing anything. We didn't understand how it worked. Well, we thought the amount available was on the strip on the card but apparently it's in the restaurant's system. Anyway, when we went to use the card just 2 weeks later, there was only $40 left on it. Someone who worked at the restaurant nailed us for another $20 in addition to the cash tip we left and of course given that we left the paperwork there they obviously knew we would have no way to prove it. Lesson learned, take your receipt when you pay with a gift card card because the restaurant staff will skin you whenever possible!