Citi Trader Made Error Behind Flash Crash in Europe Stocks (2)

Citigroup Inc.’s London trading desk was behind a flash crash that sent shares across Europe tumbling, dealing a fresh setback to the bank’s yearslong efforts to improve controls.

A trader at the U.S. firm made a mistake “inputting a transaction,” Citigroup said late on Monday after a knee-jerk selloff in Swedish stocks in five minutes wreaked havoc in bourses from Paris to Warsaw. The bank said it identified the error “within minutes” and corrected it.

Read: Five-Minute ‘Flash Crash’ in Nordic Equity Markets Jolts Europe

The violent reaction saw the main European index lose as much as 3%, wiping ...