At approximately 18:25 UTC, Bitstamp's trading engine reported a matched market order approximately 7% below the highest bid.
In an optimal matching scenario ~4,000 Bitcoins were needed to strike the price of 633.64. However, the volume at 5 minute interval was only 89.27 Bitcoins.
Something is seriously wrong with Bitstamp's matching engine, and apparently it's not the first observation by the community.