The S&P 500's second consecutive 1%+ decline has put the index 2.5 standard deviations below its 50-day moving average, which is well into extreme oversold territory. As of the close, 29% of the stocks in the S&P 500 were above their 50-days. At the lows this summer, this reading got into the single digits.
As shown below, all ten sectors are now oversold as well, with Consumer Staples and Utilities at the most extreme levels. Just two sectors were oversold at this time a week ago, so things have fallen apart pretty quickly.