The British pound sell-off and the flash crash

The British pound had one of its biggest tumbles ever Friday, with the currency sliding 6 percent in just a couple of minutes. That's a huge move for a global currency, matched only in recent times by the pound's fall after the June vote to leave the European Union. No one is quite sure what happened, with speculation running from the algorithms of trading computers, to comments from French President Francois Hollande, who said that Britain must "suffer" for its Brexit vote as a deterrent to other members of the European Union.

Flash crashes are not the norm, but they can have an outsized effect. Here's is a handful of flash crashes that have left global markets rattled:

— In 1992, the British pound suffered dramatic losses as it crashed out of a fixed exchange-rate system that was then operating in Europe. The government then abandoned the so-called Exchange Rate Mechanism. Britain has a floating currency now.

— The Dow Jones index in New York dropped 1,000 or so points in a matter of minutes on May 6, 2010. Several potential causes were cited for that crash, including a computerized sell-off possibly caused by a simple typographical error.

— On Oct. 15, 2014 the yield on the benchmark U.S. 10-year note slumped from 2.20 percent to below 1.91 percent in early trading. That's a move of 29 basis points, which typically takes weeks to occur.

— The Dow Jones index tumbled more than 1,000 points in the opening minutes of trading on Aug. 24, 2015 in part of what would turn out to be a global wave of selling that day, triggered by increased signs of an economic slowdown in China.