Benjamin Graham was well prepared for the Crash of 1929. The now legendary investor had hedged his bets: he would buy preferred stock in a company and sell short common stock in the same company. When stocks crashed in October 1929, common shares fell much faster than preferreds, and Graham made a lot of money off short sales.
But after the crash, most of those preferred shares seemed so cheap that Graham couldn't bear to part with them, he wrote in his memoirs. They kept falling, and his profit soon turned to a loss. His fund (equivalent to a modern hedge fund) ended the year down 20%. In 1930 it dropped 50.5%; in 1931 16%; in 1932 3%. "The stock market," as Graham resignedly put it in the first edition of his book with David Dodd, Security Analysis (1934), "is a voting machine rather than weighing machine."
It had actually begun voting along with Graham by then his fund gained 50% in 1933, and he did spectacularly well for himself in the next two decades. "In the short run, the market is a voting machine," he later took to saying, "but in the long run, it is a weighing machine." Over time, Graham's strategy of buying stocks that seemed inexpensive relative to a company's underlying assets and earnings really was (and presumably still is) a profitable strategy. But for months and even years on end, cheap stocks are perfectly capable of getting cheaper.
It's an important lesson to remember these days. Stock prices have dropped a lot, so stocks look a whole lot cheaper than they were just a couple of months ago. By some but certainly not all measures they even look cheap in historical terms. But that's no guarantee prices won't keep dropping.