Bad answer:

I work too hard or for the comedian, Blondes.

This question is a great opportunity to put a positive spin on something negative, but you do not want your answer to be cliche, joking or not. Instead, try to use a real example of a weakness you have learned to overcome.

Good answer:

I have never been very comfortable with public speaking, which as you know, can be a hindrance. Realizing this was a problem, I asked my previous department if I could enroll in a speech workshop. I took the class and was able to overcome my lifelong fear. Since then, I have given a lot of safety presentations to school children across the county. I still do not love it, but no one else can tell.