Unusual activity in Apple also occurred on Feb. 10 when shares dropped by more than 2 percent in just minutes before recovering, in another example of unexplained stock volatility that has caused headaches to regulators and investors.

The latest sign of abrupt volatility comes almost a year after the May 6, 2010, "flash crash," which wiped out nearly $1 trillion in market capitalization from the U.S. stock market in a few minutes.

( CNBC )