"Cisco's (NasdaqGS:CSCO - News) shares lost 16 percent to $20.57 after the Internet network product provider's CEO, John Chambers, cautioned about "short-term challenges" in Europe and public-sector spending. Late Wednesday, the company forecast revenue and earnings well below estimates.

On a percentage-loss basis, if Cisco closes at that level, this would be the worst one-day percentage drop since July 14, 1994, when Cisco slid 17.71 percent, according to Thomson Reuters Datastream.