A Japanese day trader named Takashi Kotegawa, trading under the alias "BNF," turned about $13,000 into more than $150 million over roughly eight years. His single biggest day came in 2005, when a broker's "fat-finger" error at Mizuho Securities flooded the market with erroneous sell orders — Kotegawa bought into the chaos and made an estimated $20 million before the day was over.