In 1931, the United States and the world seemed to be in a recovery mode, following the 1929 crash, and even President Herbert Hoover honestly though, "Prosperity is just around the corner." That was derailed by a seeming random event; the collapse of the Credit Anstalt bank of Austria, a country that was most noted for being just "offshore" Germany, then the world's second largest economy (now the fourth). We ought to be watching out for a similar event now.
As reported in my (Wiley, 2004) book, "A Modern Approach to Graham and Dodd Investing": "The seminal event [in 1931] that turned the global retreat into an economic collapse was the collapse of the Credit Anstalt Bank in Austria, which plunged Europe deeper into a decade-long depression (and brought Hitler to power in Germany), while infecting the United States."
There are two places where there could be a "Credit Anstalt Bank" collapse. One is in Europe, where the Greeks (rather than the Austrians) now have their hands full with the Germans. The other could be in East Asia, where China is the new "Weimar Germany," with the world's second largest economy, and is a country that is worrying about the fall of its stock market and economic growth model, and may be inclined to take extreme measures to "sauve qui peut" (save whatever it can).