"It was horrific timing to have it both happen at the same time," said Liz Ann Sonders, chief investment strategist at Charles Schwab & Co., "because investors ... were already skittish about what they're seeing in Greece."
Doug Roberts, the chief investment strategist of ChannelCapitalResearch.com in Shrewsbury, N.J., said that glitch or not, investors may have been looking for an excuse to get some of their money out of the market.
The Dow has been dropping since Monday as investors continue to worry that European efforts to prop up Greece's failing economy won't be enough and that the country's woes will reverberate around the globe.
"In a nutshell, Greece is spooking investors about all of Europe and the consequences of that for the U.S. recovery," said University of Maryland Professor of Business Peter Morici.
Art Hogan, chief investment strategist at Jefferies & Co., criticized Europe's handling of the Greek debt crisis so far.
"This is the cost of European reaction," Hogan said. "They have dragged their feet."
He added he was also shaken by the mid-afternoon plunge: "That was probably the most disturbing trading activity anyone is ever going to see in our lifetime."
The European Central Bank, he said, hasn't been aggressive enough or moved fast enough in adjusting its monetary policy to combat Europe's economic woes.
Greece is not alone; Portugal, Italy, Ireland and Spain have all faced major debt problems that have rattled financial markets. As a group, the countries are commonly referred to as "PIIGS."
The euro sank to a 14-month low below $1.27 today. Gold, often considered a safe haven in rough markets, briefly rose to more than $1,209 per ounce.
Greece's finance minister said Thursday that a joint aid package from the European Union and the International Monetary Fund was the country's only hope of avoiding bankruptcy.
For now, the country has imposed austerity measures, such as cutting salaries and pensions and raising taxes to qualify for the pacakge. The public reaction to the cuts, however, has been one of outrage and violence, with three people dying during riots in Athens on Wednesday.
With reports from ABC News' Zunaira Zaki, Daniel Arnall and the Associated Press.