I wrote $6 tip (18%) for a steak & eggs and a beer. The bartender, instead, entered $20 tip for himself. This number, I thought, he might have made a mistake while entering the tip and reading the wrong number (there was no number on the receipt that he could have mistaken). The only reason I could think of is straight up fraud. My advice is to make sure you keep your receipt and double check the charges after the payment is processed. My bill was light but it could be a bigger issue if you have a higher tab.