You might assume that as the second richest person in world with a net worth of $107 billion, Bill Gates doesn't have a lot of regrets. But there is a big one that still eats away at him, he says.

Gates admitted his "greatest mistake ever" was allowing Google to develop Android — one of Apple's biggest smartphone competitors — before Microsoft could develop a competing mobile operating system, he told Eventbrite co-founder and CEO Julia Hartz Thursday at a Village Global event.