"The trend is your friend." That is an old saw on the market, and it has made many rich. In the hurly-burly of the dealing room, clear trends - or at least trends that are clear if you can look up the chart on a Bloomberg terminal - give traders something to hold on to. There is an unfortunate corollary. When the trend breaks, you have nothing to hold on to. That is when panic ensues.

That is what happened on Thursday morning in the markets for government bonds, led by US Treasuries. It led to a wave of selling that threatened to change the underlying mathematics on which much of the world's financing is based. It also put the bull market in global stocks, now in its fifth year since equities began to recover from the bursting of the tech bubble in 2002, to one of its stiffest tests.