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.