Dealing w/ Unsatisfied Customers?

MoOsEb0y wonders: "At the company I work at, we have set up a series of SLAs giving a list of things they expect our products to do, that we promise we will deliver. In my particular situation, I have a customer who claims that the product we delivered them was slow and unresponsive. However, when we tested it to try and determine what was wrong, we didn't find anything wrong with it. How do you deal with a customer who is bent on assuming that you are incompetent, and that he or she could never have unreasonable expectations?"