2010 Flash Crash
The Dow Jones Industrial Average (DJIA) suffered its worst intra-day point loss, falling almost 1 000 points before it attempted recovery.
Image source: imgur
This was a trillion-dollar stock market crash which lasted 36 minutes. The market rebounded rapidly afterward, but the shockwave had long-lasting effects on trading dynamics. A report later described it as one of the most turbulent periods in the history of financial markets, certainly of the NYSE, but of markets generally the world over. It is interesting to note that flash crashes happen quite regularly, but measures installed after such events usually have little effect on preventing them later.