Their fears started with the “flash crash” of May 2010, when, in a matter of 30 minutes, major US stock indices fell by almost 10%, before recovering rapidly. Then came the “taper tantrum” in the spring of 2013, when US long-term interest rates shot up by 100 basis points after then-Fed Chairman Ben Bernanke hinted at an end to the Fed’s monthly purchases of long-term securities.