On June 12, 1928, the New York Stock Exchange sees a then record 5 million shares traded and a sharp decline in the midst of a market wide sell-off. Just 10 days after the NYSE had hit a record high, investors sold-off shares in droves at what the Wall Street Journal reported as "frightened selling."
Shareholders from across the country submitted sell orders fearing their shares were over-priced and prime for sharp losses. A record 803 different stocks were traded, with a record 2,082,500 shares trading hands in the last hour of market activity alone. The market-wide panic caused the ticker tape to run behind by two hours, only compounding investors' panic.
Many historians see this as the greatest pre-cursor to the Great Crash of 1929. The Federal Reserve took notice of how quickly the market could turn and implimented bans on loans for margin trades and announced an increase to interest rates in an attempt to cool the over-heated market place.
However, as we know today, the bull market continued and the market's hypergrowth led to the Stock Market Crash of 1929. (To learn more about the crash, read The Crash Of 1929 - Could It Happen Again?)