“The markets in a panic are like a country
during a coup, and seen in retrospect that
is how they were that day”
– Michael Lewis

Image: Bloomberg

October 19, 1987 is famously known in history as the day Wall Street crashed, or in other words “Black Monday”. To this day it remains the biggest single one day stock market drop in history - to be more precise a drop of 23 percent.  
It is thought that the crash was caused due to computer driven trading models that followed a portfolio insurance strategy as well as investor panic. Portfolio insurance was created to save investors from market declines but ironically became the downfall. Stockbrokers found themselves in dread and world markets went tumbling down. Black Monday had an enormous influence on investors, regulators, and exchanges.
Since October 1987, several protective mechanisms have been built into the market system to prevent any such occurence in the future.