Remember, the interviewer isn't here to condemn you. Provide an honest answer. With that said, try to stay away from a severe deadline you may have missed in your life. The interviewer is looking for a simple deadline, and what you learned from your mistake. Did you simply under estimate your time? Did you face a distraction at work? Were you in over your head? Whatever the situation was, you had to have learned from the outcome, and it had to make you better in someway. Explain that to the interviewer.