I've always believed that a tip represents service provided. And that doesn't always boil down to efficiency. If my waitress forgot to bring me a spoon for coffee or ketchup for my burger, I won't penalize her if she's genuinely pleasant, on the ball and doing her job. Everyone makes mistakes. But if she's dragging along and rolls her eyes while tossing the forgotten spoon at me, why should she be rewarded with a generous tip? I feel obligated to let her know that her attitude sucks.