Candidates who can't think of a mistake or learning experience don't impress. This question establishes whether the interviewee is self critical and has the willingness - and humility - to keep learning. Don't say nothing because this makes you look arrogant and lacking in imagination. Don't say that your biggest mistake is being a perfectionist - this appears contrived or insincere. It's good to give examples that show you can accept failure and admit your mistakes. Name a situation that didn't work out well or a task in which your team fell short.