When a Trading Glitch Costs Billions: Lessons from Japan’s Mizuho Fat‑Finger Disaster
A 2005 fat‑finger error by a Mizuho trader triggered a ¥400 billion loss, leading to a landmark court case that clarified liability for software bugs in financial systems and highlighted the need for rigorous testing and evidence preservation.
Background
In September 2005 a trader at Mizuho Securities mistakenly entered a sell order for 61 万 shares of J‑Com at ¥1 per share instead of a buy order for one share at ¥61 万, causing a massive “fat‑finger” error.
Immediate Impact
The erroneous order was accepted by the Tokyo Stock Exchange (TSE) system, the price warning was ignored, and the order executed, driving the opening price to ¥672,000 and pushing the stock to its daily limit down to ¥572,000. The mistake resulted in a loss of about ¥400 billion (≈¥27 billion CNY) for Mizuho.
Aftermath and Settlement
Mizuho attempted to cancel the order three times but the TSE system rejected the cancellations due to a hidden bug. The exchange refused to intervene, and the trade was settled in cash at ¥910,000 per share, increasing the loss to over ¥400 billion.
Legal Proceedings
The dispute went to court. The Tokyo District Court held the TSE primarily responsible, with Fujitsu (the system vendor) as a joint liable party. The court ruled that a bug is not “gross negligence” unless it is easily detectable, and assigned 70 % of the loss to the exchange.
Broader Implications
The case established that the detectability of a software bug will be a key standard in future litigation, prompting stricter testing, evidence‑preservation, and modernization of legacy trading systems.
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