Hmm, I kinda disagree with PPs. I think this is different from the "biggest weakness" interview question, because the interviewer doesn't know your weaknesses, whereas your manager probably does know what mistakes you've made (and no one gets through a full year without making ANY mistakes.
As a manager, I'd much rather have my employees acknowledging their mistakes than pretending they didn't make them. Don't throw yourself under the bus, but come up with a deadline you missed, or a conflict you didn't handle as well as you could have, or a meeting that you weren't sufficiently prepared for, or maybe just a training/advancement opportunity you didn't take advantage of, and explain how you will handle similar situations in the future.