Dow falls hard - Due to Human error?

Dow closed 348 down. But in intraday trades the market hit 998 points down before recovering. Typical reaction was something must have gone wrong with the system. An explanation given is that there were technical glitches. Another story doing rounds is that a trader placed a huge sell order by mistake when he actually intended to put a huge buy order.

Really?

Wonder why do these glitches/error occur only on one side. Ever heard of a glitch causing markets to rise 1000 points in a day. Wonder whether last year's 17% rise in Nifty in a single day was due to human/system error!