High-frequency traders are routinely distorting stock and futures markets by illegally acting as buyer and seller in the same transactions.
Wash trades, are banned by U.S. law because they can feed false information into the market and be used to manipulate prices. Intentionally taking both sides of a trade can minimize financial risk for the trading firm while potentially creating a false impression of higher volume in the market.
Average Joe took "sell in May" a little too seriously today. In a few weeks, once things calm down and rebound, he'll start whining and crying for the remainder of the year that Wall Street is corrupt and the market is stacked against small investors.