"You had the backdrop of the unrest in Europe, the fear in the market, then we saw some heightened activity in the futures market that spilled into the equity market and what happened with Proctor & Gamble, and stocks such as that -- Accenture -- is we saw the main listed market basically step away, not answer the electronic phone calls, that also spooked the market and that was the additional factor that helped drive us into that abyss," he said on "GMA."
McSherry said until the economic situation in Greece is stabilize and until European leaders step up and address the crisis, the markets are unlikely to end their volatility.
"Until European leaders can step to the floor and show that they've really got their arms around this problem, are willing to address this forcefully, we're going to see days like yesterday," he said. "We're not going to really see us calm until that happens."
For investors nervous about the markets, experts say they should look at their portfolio and gauge how much risk they can comfortably take.
"It all goes down to what the risk profile is of each individual investor. There's no one right answer that covers all investors," Liz Ann Sonders, chief investment strategist at Charles Schwab, said on "GMA."
If the situation is going to dampen one's emotional stability, then they probably have too much risk in their portfolio, but those who are taking more longer-term investments may be able to take more risk.
"It really depends on where that investor sits on that risk spectrum," she said.
The situation, and especially if it was caused by human error, is unlikely to help smaller investors who feel overshadowed by larger firms to begin with.
"Even prior to yesterday, I think if you were to ask a lot of individual investors, they did feel like the market had become rigged against them and that it was a bit of a game that they couldn't possibly play with increased volatility," Sonders said, and if it was human error, which hasn't been confirmed yet, it won't "help improve the psyche of the individual investor."
Around the world, markets followed the lead of the United States and dropped significantly.
Asian stock trading got off to a tough start, with major market average turning in big red arrows in early trading. Fears of global contagion from the European sovereign debt crisis reverberated across the globe. While the most significant of the U.S. market's plunge were dismissed as a result of some unattributed trading error, there was a belief that the better than three percent sell-off at the closing bell wasn't a fluke.
The American markets, however, are on target for a positive open, according to the futures markets. The latest data -- a good predictor of the momentum -- shows the Dow opening up about 10 points, the broader S&P 500 up 2 points.
ABC News' Zunaira Zaki, Matthew Jaffe, Rich Blake, Ned Potter, Bianna Golodryga and Simon McGregor-Wood contributed to this report.