Today the S&P 500 traded down to a low of 1076.12, that's 8.2% below (3 standard deviations!) the 50-day moving average. In February the S&P reached similar levels intraday on 2/5, and then again on 5/6, but each time the market rallied back to close above that 3 standard deviation threshold.
The last time we closed this low was during the Lehman Brothers sell-off in 2008, but there is a fundamental difference between then and now. When the market began to fall apart in September 2008 we were already in a clearly defined downtrend (shown in the charts below). Currently the S&P is in a very clear and significant uptrend. When using these overbought and oversold tools momentum is hugely important. In any oversold market it is nearly impossible to call the bottom, but the case for a significant bounce back to the 50-day average is very good.