There was a very expensive stock-trade mistake when someone issued a sell order, typing 'b' instead of 'm' for 'million'. Market dropped a significant fraction in seconds.
Here is one of the "b" instead of "m" stories published on the day of the crash:
http://www.cnbc.com/id/36998463
Here is the Wikipedia summary of the event which lists "fat finger trade" as one of the discredited theories:
http://en.wikipedia.org/wiki/2010_Flash_Crash#Early_theories