I've only once ever not left a tip. I was eating with two friends at a smallish bar and grill. Since I was driving, I only ordered a soda with my meal, which drew some unprofessional remarks from the waitress. She then took our food order twice, which should have been a clue as to how drunk she was, but we didn't realize it until she delivered our food to the wrong table. Near the end of our meal, we ordered one more round of drinks. When the whole drink order came back completely wrong, I made the mistake of asking her if she was feeling ok. She proceeded to drop a slew of racial slurs on one of my dining companions. When I spoke to her manager, he defended her, and threatened to kick my ass. We walked without even looking at the bill.