Not doing something you have always wanted to do before dying; that would be my answer because it would be my biggest failure. I think a fair answer because: you're not dead yet, which means you may have not had your "biggest failure". Best way to approach this one I think is to think big >_< I may have twisted the answer just slightly, but it's up to them if they decide to press the question and ask for a "biggest failure" of your past; highly unlikely they would do that, they never rephrase questions that are that general. Man, what a depressing request to answer -_-