A trader at the U.S. firm made a mistake “inputting a transaction,” Citigroup said after a knee-jerk selloff in Swedish stocks in five minutes wreaked havoc in bourses from Paris to Warsaw. The violent reaction saw the main European index lose as much as 3%, wiping out 300b euros at one point. It revived questions how large financial firms can prevent such errors, and whether markets have sufficient safeguards in place.