You need to demonstrate how you dealt with a problematic situation, turned it into a learning experience, and avoided repeating the mistake. Use the STAR method (Situation/Task, Approach, and Results) to prepare your story, detailing what you learned from your experience. Everyone makes mistakes, so don't say that you have never failed.

"In my past position, I was responsible for managing a project for one of our biggest clients, and I was so eager to please them that I told them we could finish the project within two weeks. I thought this was doable, but it took twice as much time, and the client was not happy. I should have been more conservative in my estimate to the client. I realized that a client isn't going to be upset if you're clear about the timeline in advance, but they're going to be disappointed if you promise something and then don't deliver. I used this experience to learn how to manage my clients' expectations. For example, on my next project with a different client, I told them it would take four weeks to complete the marketing campaign, and we were able to finish in three weeks, ahead of schedule - making the client very happy."