Uh, that isn't true at all. I think what you are thinking of is that the Silk Road server's IP was allegedly exposed when the captcha system leaked it's true IP. There was no bug in Tor or Tor browser, simply a bug in a piece of code that Ross wrote. It wasn't the first time a bug he introduced leaked the Silk Roads IP either:

https://www.reddit.com/r/SilkRoad/comments/1dmznd/should_we_be_worried_showing_on_login_page/