Electronic trading systems allow trades to be executed rapidly and efficiently, but also make it easy for traders to make errors by punching in the wrong numbers particularly while trading in a hurry. This case describes a large trading error at Mizuho Securities that cost the firm $335 million as well as two other similar incidents in India and in the United States. The case describes the deficiencies at the trading firm and at the exchange that lead to such mishaps and also discusses how traders and exchanges could respond to these errors.