"In one second more or less someone (and yes, under these circumstances, human beings take control of the machines) made the decision to pull the bids on every equity in the S&P,  every financial futures contract, every FX contract in every market in the world.  This kind of thing just doesn't happen in a pure auction environment; there just isn't a tight enough communication link between the parties to allow the decisions to propagate within the same second -- even with HFT algorithms.  No.  Some human made the decision to pull the bids; all of them, all at once.  If that is not a condemnation of the concentration of financial power and the systematic risk it engenders I don't know what is"  Full article