Only one thing? But that would alter my entire life wouldn't it? I think it was dumb selling my first building I was part owner of. I would be well off now if I had kept it... but then my life would be different in so many other ways. I wouldn't have run off to England and missed all that adventure and the many that followed.
I try not to regret anything but there are lots of things I wish I had done differently... in hindsight.