looks like it is impossible to do. For instance, if some one bought 10000 shares of PG at the low at $39, and sold them at $42 5 seconds later. Now NASDAQ busted the trade of 10000shares buy at $39, then it would leave that trader 10000 shares of short of PG at $42, now that trader has to cover at $60. So $30K profit for that trader turned into $180K loss. I do not think this is fair at all. from 2:40pm to 3:00, I am sure many traders were in and out many rounds, especially for those High frequency trading big guys. Now all of their buy trades were busted, and left millions of shares of short. How could they cover?