Mizuho Securities Co., Ltd is a Japanese investment banking and securities firm which is a part of Japan’s second largest bank, and they take the top spot for losing millions because of a very simple mistake. In 2005, the firm added the recruitment company J-Com Co., Ltd to their economic portfolio, and when one of their traders went to sell some of the new acquisition’s shares on the Tokyo Stock Exchange, he made a simple yet monumental mistake. The trader was supposed to sell one share for 610,000 Yen, but instead, thanks to a typo, he sold 610,000 shares for just a single Yen. Now, you do not have to be good at math to know that that was a huge and terrible blunder, which it was, because as a result of that typo, Mizuho Securities ended up losing approximately 340 million dollars in just one day.