Citi fined $78 million after a European trader's fat-finger trade caused a flash crash in stocks

• A trading mistake at Citigroup in 2022 has led to a $78 million fine against the bank.
• The "fat-finger" trade caused a brief flash crash in European stocks in May 2022.
• The brief flash crash caused by the mistaken trade briefly wiped out $322 billion in market value.

A trading mistake at Citigroup has resulted in a $78 million fine.

A trader in Citigroup's London unit entered one too many zeros in a trade in May 2022, sparking a short-lived flash crash in European stocks.

The trade itself was entered during early European market hours and caused a five-minute flash crash in the OMX Stockholm 30 Index, wiping out as much as $322 billion at one point.

According to Bloomberg, the trade was meant to hedge the bank's exposure to the MSCI World Index.

While the trader meant to execute a trade that would create a basket of stocks valued at $58 million, they accidentally entered 58 million into the quantity field, ultimately creating a massive trade worth $444 billion.

Citi's trading systems fired off warnings and prevented much of the trade from going through, but not all, with about $1.4 billion worth of the trade being executed, resulting in a cascade of sales of European stocks across various European exchanges.

UK regulators investigating the trading blunder leveled a $78 million penalty against Citigroup for the mistake on Wednesday. 

"The immediate cause of the trading error was a manual input error by the trader. The error was then not identified by either of the firm's risk functions dedicated to real-time monitoring of the firm's trades, but by the trader some 15 minutes after the trade was entered into the firm's systems," the Bank of England's Prudential Regulatory Authority said on Wednesday.

The $78 million fine from UK regulators is in addition to about $50 million Citigroup lost on the trade, bringing the total cost of the fat-finger trade to about $130 million.