How to Avoid a $444 Billion Input Error
Fixing the UI that crashed the stock market
In 2022, a Citigroup trader made the mother of all input errors when selling a basket of equities valued at $58 million. The trader entered the value in the unit quantity field instead of the notional value field. As a result, Citigroup crashed the European stocks and received a fine of $79 million (source).
A better form interface could have prevented this input error. The form had two fields for selling stock. Traders could enter the number of shares or the dollar amount they want to sell. The trader who made the input error entered the dollar amount in the number of shares field. Instead of selling $58 million worth of stock, they accidentally sold $444 billion.
