As with any business, restaurant patrons will sometimes have complaints and it will likely be your responsibility to resolve them. Perhaps a steak was underdone; perhaps another member of the wait staff was rude or unkind to the customer. Whether or not the example you can provide is directly related to the restaurant business is often irrelevant. For example, "When I worked in retail sales, I once assisted a woman who had purchased a defective product. Although she had to go through the manufacturer for a replacement which made the process difficult, I walked her through it each step of the way to achieve a resolution" is a perfect answer.