Case Study: The 2005 Mizuho Securities Fat‑Finger Trading Error and Its Legal Aftermath

In September 2005, a trader at Mizuho Securities mistakenly entered an order to sell 61 million shares of J‑Com at ¥1 per share instead of buying one share at ¥610 000, triggering a massive sell order that caused the stock price to plunge and resulted in a loss of over ¥400 billion (≈ ¥27 billion RMB).

The exchange’s trading system, developed by Fujitsu, displayed a warning dialog that the trader ignored, and subsequent attempts to cancel the order were rejected due to a hidden bug in the system’s code, preventing the reversal of the abnormal trade.

The erroneous trade disrupted the market, leading to chaotic price movements, forced buy‑backs by Mizuho, and significant losses for other securities firms. The incident also caused a broader market panic, with the Nikkei index dropping sharply.

After the event, Mizuho sued the Tokyo Stock Exchange and Fujitsu. The Tokyo District Court found the exchange primarily responsible for the failure to halt the trade, while Fujitsu was deemed a joint liable party. The court ruled that the bug did not constitute “major negligence” but that the exchange’s failure to act was a serious breach, assigning it 70% of the post‑call losses (≈ ¥107 billion).

The case set a precedent in Japanese law: the ease of detecting a bug becomes a key factor in determining “major negligence” in future software‑related litigation. It also highlighted the importance of rigorous regression testing, bug‑density metrics, and evidence preservation for software development teams.

Ultimately, the incident spurred the Tokyo Stock Exchange to modernize its trading infrastructure, leading to the development of the next‑generation “Arrowhead” system, again contracted to Fujitsu, emphasizing the need for reliable, well‑tested systems in high‑frequency financial environments.