No matter how skilled you are, there will always come a time when a customer will be dissatisfied with something you have prepared. Whether or not you feel that you made an error, the customer is always right. You should answer with "I would listen very carefully to the complaint and resolve the issue by anticipating the needs of the customer. I may ask questions to discover the nature of the complaint and take the proper steps to ensure that the error does not occur a second time." No matter how serious or slight the issue, there is always something to be learned. Your interviewer will want to make sure that you will not take small complaints too seriously or more complex complaints too lightly.