No one was sure what happened, other than automated orders were activated by erroneous trades. One possibilility being investigated was that a trader accidentally placed an order to sell $16 billion, instead of $16 million, worth of futures, and that was enough to trigger sell orders across the market.
http://news.yahoo.com/s/ap/us_wall_street
http://news.yahoo.com/s/ap/us_wall_street
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