The sudden plunge in the US markets on May 6 has again focused attention on algorithmic trading and high-frequency trading.
Whatever caused the initial trading, accidents can always happen (and no doubt many people will spend a great deal of time dissecting the events of May 6 - to determine exactly what did happen).
However it is clearly vital that the market structure has an efficient way to deal with any such problems (whether the cause is human or machine).
It looks like the SEC and execution venues are looking at more consistent ways of dealing with this (as highlighted in advancedtrading.com). If volatility auctions had been consistently triggered across the venues for any stocks where there were large sudden price swings - then the calamitous price drops may well not have occurred.