Buffett's speech at Stanford Law School in 1990 is a classic.

He said that he and his wife Susie each kept one share of stock. He bought it in 1955. At the time, the company seemed to have good quality, stable business, and appeared very safe.

The only problem was that its core assets were in Havana, Cuba.

Later, when Castro came to power, the assets were confiscated, and this stock basically became worthless.

Buffett said the reason he kept this stock was to remind himself of one thing:

In some countries, the rules can change overnight.

His exact words were:

“The rules in other countries can just change overnight.”

What investment fears most, often, is not misjudging a company, but misjudging the rules.