What has been your biggest mistake?
abstractbill: I used to have regrets.  For much of my early 20s, I thought not having tried to make music professionally was a mistake.  Then, later, I worked at Macrovision and saw the music business up-close and was very glad not to be part of it!  Similarly, for a couple of years after my PhD I regretted not staying in academia.  Working for a startup has completely cured me of that regret.I guess I've learned that something you believe to be a mistake today can turn out to have been a smart move given a year or two's hindsight.