If you are talking about the US "Flash Crash" that theory is little more than urban legend.

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