Interviewer: What is the biggest mistake you made in your career life?
Me: (Answered with the biggest mistake I made, what I learned and what I should do for better result)
Interviewer: Umm.... That mistake sounds not big enough...
Me: That IS the biggest mistake I have ever made. If you really want to see a big one, let me join your team and I can make one for you.

So I guess they want someone who made severe mistake?