Fig. S&P500 futures, 2011-12-08 / 09; source: XTB
The S&P500 futures have fallen from 1270 to 1222 yesterday (i.e. -3.8%), to bounce back to around 1250 today (+2,3%).
"The European Union will die - SELL, SELL, SELL!" - suggested reaction to the yesterday's ECB announcement. "The European Union is saved - BUY, BUY, BUY!" - declared investors after the summit.
I have seen quite similar behavior somewhere, some time ago...