You can tell my greatest weakness is I can't say no. Scenario- There have been times where I was assigned to the task and my colleague came for help when I had the task due on the same day. So, I started helping my colleague and since my colleague was new on the project had many questions. I ended up spending a couple of hours answering the query of my newly hired colleague. This did affect me as I had the assigned task due on the same day and had to stay extra to finish my task. It is hard to say no right away even though I am middle of something.

I hope this helps, there is no right or wrong answer for the behavioral question. Please keep in mind that you need to show that you had weakness but you have done something to overcome the weakness. Weakness shouldn't really be your weakness and should tie to your strength or something good.

Good Luck for your interview.