The 15 Minutes on the Citi Trading Desk Behind Flash Crash (1)

For one Citigroup Inc. trader in London, the morning of May 2, 2022 went from bad to worse.

It was a public holiday in the UK, so it should have been a quiet day in the markets. A little before 9 a.m., the staffer on Citigroup’s Delta One trading desk — who was working from home — began putting together a trade that would hedge the bank’s exposure to the MSCI World Index.

A tool that employees usually used for such a transaction wasn’t available that morning, so the trader had to manually build the basket of stocks. That’s ...