Wall Street Crash:
During the 1920s the US stock market rose dramatically, taking prices beyond anything which reflected likely long-run value. The result was the 1929 crash. The loss of confidence which this caused was a major factor in the 1930s depression. From top to bottom, share prices fell by 89%.
A similar but less dramatic cycle, where a long period of rapid share price increases was followed by a correction which frightened people into selling all at once, has occurred a few times since then. Black Monday in the UK and the US in 1987 marked one such event; some Asian countries experienced the crashes associated with the Asian financial crisis in 1997–98. Some share indices fell by about half during late 2008.
They rallied later but that may not be the end of the story.