Turning a bad experience into a positive one

I ordered my meal and waited 20 minutes before the waiter came back to tell me it was nearly ready, but they didn't have what I asked for so wanted to replace one of the main ingredients. I was not happy with this and the waiter went to get the manager, who offered me anything I wanted, on or off the menu, complimentary. I was happy to pay for my meal, but really appreciated that he was willing to go out of his way to fix things. Excellent service.