It's interesting what happened that day. At the opening of FB stock, Nasdaq tried to match about 75 million shares' worth of buys and asks--the 'opening cross'. However, if any one of those trades were updated, Nasdaq's engine was programmed to partially or completely throw out the cross and try again, resulting in an infinite loop since they couldn't calculate the cross fast enough every time there was an update. This is a pretty basic mistake in a system designed to process lots of events in real-time.

Nanex always likes to cast HFT as the villain of everything that goes wrong with the market--this is silly. HFT makes nothing but money for Nasdaq, and narrows spreads for investors. It was pretty clearly Nasdaq's job to handle the volume of information and not falter due to such a dumb mistake as other networks kept chugging along.

The same bug also affected trading in Zynga later that day on lower volume when trading closed and re-opened on that stock. This to my mind makes Nasdaq look guiltier. It's like they didn't even test what would happen handling a big cross under load.