The only time I recall giving less than 15% was about 5 years ago. My wife's business partner and I went out for dinner while my wife was out of town visiting her grandmother out of state. We showed up at 10 PM for a late dinner at a usually nice upscale diner (we'd been there 3 or 4 times before with no problems). We were seated alone in a large dining area. There was one (1) waiter for the area, so we expected reasonable service. Apparently the fact that we were dressed less than impeccably (we had been hauling and loading things for my wife's business) led the waiter to blow us off. We waited 10 minutes to place an order, then 15 minutes for appetizers and drinks to show up. The waiter just didn't seem to want to deal with us.
After about half an hour with no drink refills (water and Pepsi, for Pete's sake!), a party of six nicely dressed elderly people came in and was seated at the table next to us. Oh boy did that waiter play a different tune for them: helping ladies to their seats, smiling, helping with coats, passing out menus himself, chatting all of them up, and never once looking our way. I muttered to my friend, "Cripes, what does it take to get a refill around here?" As it happened, the waiter was right there and shot me a dirty look. He went away with the other table's drink order and returned with our food, which he dropped unceremoniously in front of us. The food had obviously been sitting under the heat lamps for a while, and was by no means good at all. We finished off our lousy food while the waiter put on his floor show for the nice older folks. I had to snag him to get our check, which he dropped off immediately. We were both so steamed that, when I paid the bill at the register, I deliberately wrote in enough of a tip to round the bill up to the nearest dollar on the charge slip, giving the guy less than 50 cents on a bill of about $15.00. The cashier gave us a sarcastic "Thanks!" as we left. Needless to say, we have never gone back to that place again.
Yes, we could perhaps have spoken to the manager, but by the time we got to the cashier we were both so insulted and indignant that we couldn't muster the gentility necessary to inform the manager of what had happened.