Use an example were you were not able to meet a deadline due to outside factors.
For instance , there was a big project that my team was working on and I had split up the work among some members and myself. During that time, one member of the team had to leave due to their spouse getting a position in another city. He left at a critical time and I had to re-assign his duties to someone else. I had to get this new person up to speed with the progression of the project and due to this, was not able to complete it on time. We were still able to complete the project a few days after the deadline even with the change in team member.   _{Your Comment}