http://www.nypost.com/p/news/local/irs_says_this_broke_ny_hipster_better_YO0mGaltl4KryhYx53iU8L Guy never made money but owes more than Bloomberg/Buffett/Gates combined. Key thing is the last paragraph: The number was likely generated by the total amount of trades he made, instead of the profit he earned from them, one tax expert said, adding that's a typical move by the IRS when returns aren't filed. So the lesson here is list EVERY trade and don't be lazy.
My guess is he did not file a return and all the IRS has is the sale proceeds from all his day trading, which is 172M. When he files a return with his actual basis it will be cleared up.