DuWayne Peterson always believed in making time for business and IT strategy. In fact, on the morning of Monday, Oct. 19, 1987, Peterson, then vice president of systems, operations and telecommunications at Merrill Lynch, went to his office in the World Financial Center in lower Manhattan while a good portion of his senior staff attended a strategic offsite in Princeton, N.J. The previous Friday, to Peterson and the rest of his Wall Street colleagues, the market looked grim. The the Dow Jones Industrial Average (DJIA) fell 108 points to close at 2246.74. On Thursday, U.S. Treasury Secretary James Baker expressed concerns to the media over the massive sell-offs. Over the weekend, investors began to sweat. Pundits, economists and politicians went on television, hedging their bets about the market.

“Friday we saw trouble,” Peterson says. “But we had no idea what we’d walk into that Monday morning.”

Black Monday, as it became known, would represent the largest decline in U.S. stock market history. The DJIA dropped an astonishing 508 points (22 percent) to close at 1739. Investors sold off stock in a frenzy. Peterson says Merrill Lynch’s systems, which usually saw 100 million to 200 million shares a day traded on the New York Stock Exchange, buckled on Black Monday as volume soared to around 604 million. Very quickly, Peterson knew he’d need help.

“In the morning, my senior person called me from New Jersey and asked me if I wanted them to come back,” Peterson remembers. “But I said, ‘No, strategic planning is important. We’ll handle this.’ Well, by mid-morning, the volume just started roaring, and they had to hustle back.”

And Peterson’s situation was hardly unique. Financial services firms and their technical people worked frantically throughout that Oct. 19 to keep up with the massive volume. As Black Monday celebrates its 20th anniversary today, economists, business leaders and politicians believe that bizarre day of chaos taught the country just how profound an effect information technology—and the way humans behave using that technology—plays on the market. Technology, just like humans, can make mistakes. Systems built to withstand loads of information can be overwhelmed, and computers don’t stop and take a deep breath in times of crisis to assess why it’s happening. Twenty years later, Black Monday serves as a reminder that balancing technology-enabled efficiencies with the realities of human behavior is difficult if not impossible without key safeguards.