Thirty years after "Black Monday" — one of the worst days in Wall Street history — economists and historians are still analyzing the causes of the crash.

One factor, a new, financial strategy that had become incredibly popular, “portfolio insurance.” As it turned out, traders, investors and regulators were ill-informed about how this shiny, new tool that was supposed to minimize risk would actually perform in a market downturn.

“When you have new technology that is ill-understood, both by traders and regulators, it’s dangerous,” said Karen Shaw Petrou, managing partner at Federal Financial Analytics. “It’s like giving a six year-old boy a Ferrari. It’s really not a good idea.”

We're taking a look back at the 1987 crash, and what it says about future declines in the stock market. 

This is the second story as a part of WNYC's Crash Course, a series about the stock market crash of October 19, 1987, and what it can tell us about today's financial markets. If you've ever regretted a decision about investing in the stock market, how did you make that choice? Tell us your story.