We once had an indifferent and borderline rude service at a restaurant however; the food itself was quite good. It was my treat that day so, I paid the bill exactly as it stated on paper and omitted to leave the usual x% service. As we were leaving I was asked by the headwaiter if he could have a private word with me.
Waiter: Sir, was the food okay today?
Me: Yes, it was fine thank you
Waiter: I just wondered if you didn’t like your food
Me: I liked my food just fine; nothing to complain about
Waiter: In that case, is there a reason why you didn’t leave a tip sir?
Me: Oh, you want a tip, do you?
Waiter: Well, yes sir, this is the usual practice as it says on the bill
Me (inserting my hands in my pockets): Okay, here is a tip: in the future, make an effort to provide at least an average service but preferably a good service; then you and the other waiters might get some gratuity.