When the waiter definitely deserved their tip:

"I don’t leave tips only if I didn’t like the service at all or if the waiter was rude. But once, I was at a restaurant, and everything was superb. When I was about to pay the bill, I realized that I didn’t have cash, and the nearest ATM was about 2 miles away. I felt embarrassed not to leave a tip, so I paid my bill with the card and left without the waiter seeing me.

I was already quite far from the restaurant when I heard someone running after me. It was the waiter who cried out, “You forgot your card!” and handed it to me. And at that time, we didn’t have any apps that allowed you to pay with your phone, and I was on a business trip in this town. So, I went to the ATM, withdrew some cash, returned to the restaurant, and left him a 50% tip."