It is important to understand the client's expectations to determine if they are reasonable. When discussing an example, demonstrate how you were able to under promise and over deliver and exceed the customer's expectations during the process

"One of our recent customers recently added a fourth phone line and upgraded three phones. We had one phone in stock but the remaining three had to be shipped. When completing the order I told them they would receive them in 7 days. However, as the delivery date was approaching I realized the order wasn't going to ship for another 14 days. As a result, I called the customer, provided a detailed explanation. Most importantly I apologized for the delay and inconvenience this may have caused. The client was appreciative and received their phones 10 days later."