> The 1987 market crash (October 19, 1987, single-day loss of 23% in the S&P 500 index) was a 25 standard-deviation event.

Adding to this, the Swiss Franc revaluation in 2015 was called a 20 standard deviation event [0].

[0] https://www.ft.com/content/5a06ef16-b5e4-11e4-a577-00144feab...