Guys, just to clarify… When you give the server your card and they return with the receipt for you to put in a tip the transaction has already happened. If you live a tip they will add it in later. There is no way to input a negative number for the tip. However a discount can, at this time, be applied. What the person in this picture has effectively done is to present a homemade “100% off” coupon. No business is going to honor a coupon you made yourself and as this is no different, the transaction will simply be processed with no tip included. As they didn't actually try to pass a fake coupon, no law has been broken. If they're sufficiently douchey, they can contact their credit card company and dispute the charge, but there's almost certainty that in this instance the vendor will get paid. It's shitty, nasty, and catty, but at the end of the day it's no different than leaving a nasty note instead of a tip, nothing more.