The S&P 500 (SNPINDEX:^GSPC) fell more than 50%. Citigroup (NYSE:C) went from being the world's premier megabank to being nearly bankrupt in a matter of months. Even General Electric (NYSE:GE) found itself in a money squeeze that could have threatened its existence.

And almost no one saw it coming.

It might be the single-most important question of the last five years: Why were so many people caught off guard by the 2008 financial crisis?

Until we try to answer that, we're at risk of being blindsided by another.

Last week, I asked PIMCO CEO Mohamed El-Erian why so many people missed the financial crisis. Here's what he had to say.

Fool contributor Morgan Housel has no positions in the stocks mentioned above. The Motley Fool owns shares of Citigroup Inc and General Electric Company. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.