One time when I was a server I had a table of German tourists. They ordered a spring salad, which was based on very fresh local spring greens. When I came to check on how they were doing, how they liked the salad, they pointed out that they hadn't eaten any of it because there was a caterpillar crawling around!

Luckily they hadn't eaten the caterpillar OR the salad. Obviously the restaurant apologized profusely, took the price of the salad off the bill, and even gave them a free T-shirt. They were incredibly understanding and said "It's fine, it proves how fresh your salads are!"

When dealing with customer complaints it's always best to go above and beyond. And when being a complaining customer it's always best to remember that these are people and being understanding might mean they'll go the distance to help you!