He cites four possible catalysts for a crash.

• "A drop in oil to the $50s." WTI December contracts settled at $77.40 a barrel Monday, after hitting a three-year low of $75.84 last week. "Just another $20 drop in oil would cause tremendous pain in the entire energy sector and would quickly spread to the financial sector," Willard writes.
• "Food prices and inflation take off." Overall consumer prices rose only 1.7 percent in the year through September. But food prices are on the rise, Willard says. "Food inflation should be on your radar right now."
• "A spike in gold to $2,000 an ounce." December gold futures settled at $1,159.80 on the Nymex Monday, after trading at a four-year low of $1,130.40 Friday.
• "Currency wars." Many financial commentators say these have been going on for several years, as countries around the globe seek to boost their exports by depressing their currencies. "Currency wars create dislocations and unsustainable cycles that catalyze crashes in markets and economies," Willard writes.

Whatever Willard's worries, many investors remain bullish on stocks.

While "companies are by no means knocking the cover off the ball" in their third-quarter earnings reports, "the mood is that things are OK," Brian Fenske, head of sales trading at brokerage ITG, told The Wall Street Journal. "People still like stocks broadly as an asset class."

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