It's generally considered a negative technical signal when an equity index breaks below one of its major moving average lines. With that in mind, the fact that the Nasdaq broke down below both its 50 and 200-day moving average today has bullish technicians feeling pretty glum.
Falling below both the 50 and 200-day moving average on the same day is certainly not a common occurrence. Since 1971, there have only been five other days where this occurred for the Nasdaq. Following those periods, the Nasdaq has averaged a gain of 0.36% over the next week and a decline of 1.2% over the next month, and in each case the Nasdaq was positive three out of five times.