Getty Images Memories…It’s been three decades since Black Monday, the most disastrous single day in U.S. stock market history, and investors should be forgiven for wondering if Wall Street learned any lessons from that cataclysmic day. On Oct. 19, 1987 the Dow Jones Industrial Average DJIA, +0.18% tanked 508 points, a fall of nearly 23%, in a chaotic, daylong selling frenzy that ricocheted around the world. The S&P 500 SPX, +0.07% shed more than 20% of its value. At today’s market heights, a percentage fall of that magnitude would knock more than 5,200 points off the DJIA.via