Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

May 6, 2010 stock market decline/plunge: Why did it drop 9% in a few minutes?
No one is quite sure what happened (yet).  Speculation includes: The interesting thing is that Procter & Gamble stock got hammered, as did Accenture.  Both of which are fairly stable companies, that didn't make any major announcements, and aren't really connected to the current financial instability in Greece.  So, there is no reason for there stock prices to have gone crazy like that.  This points to some kind of screw up, and not a regular market force. Apparently, the trades involved in this event are going to be canceled. Edit #1: One thing that can contribute to an event like this is automatic selling triggered by stop loss orders.  Say someone at Citi makes a mistake and sells too much of a stock.  That drives the stock price below a certain threshold.  Computers that were pre-programmed to sell at that point start doing their job.  Now the price goes even lower.  More stop-loss orders get triggered.  Things start to snowball.  Since it's all done by computer these days something like this can happen in seconds.  All the humans are left scratching their heads. (No idea if that's what actually happened.) Edit #2: IEEE Spectrum has a pretty concise article on the topic.  It also includes some links to follow. Edit #3 (05/14/2010): Reuters is now reporting that a trader at Waddell & Reed triggered all of this, but not through any wrongdoing. Edit #4 (05/18/2010): Waddell & Reed claims they didn't do it.  The House Financial Services Subcommittee investigated, but they couldn't find a "smoking gun".  I think at this point, people have pretty much given up trying to figure out what happened. Edit #5 (07/14/2010): The SEC still has no idea.  I'm giving up. :-)