31 July 1914: The NYSE shuts its doors for months

With war spreading across Europe and foreign investors dumping American stocks for gold, the New York Stock Exchange closed to prevent a collapse. It stayed shut for about four months, the longest closure in its history, and reopened only gradually in December. Treasury officials worried about a gold drain. Investors who needed cash simply could not sell. Liquidity, it turns out, is a fair-weather friend.

The lesson: The ability to sell can disappear exactly when you need it most, so do not depend on it.