If I were to contact your most recent supervisor, what area of your work would he/she tell me you could most improve upon?

Choose an area that fits into the overall picture for the job. Focus on something that you recently learned, so you can show how you grow into tasks and what you do to improve as you acclimate. Be positive, but not overly confident.

"I think my boss would probably say I take on a bit more than I should, and I tend to get overwhelmed at times. We've discussed this, and I have developed a stronger sense of when to delegate, but I still do have ups and downs in my productivity because I occasionally over-schedule my time."