The S&P 500 (^GSPC +0.42%) fell more than 50%. Citigroup (C +1.02%) went from being the world's premier megabank to being nearly bankrupt in a matter of months. Even General Electric (GE -0.31%) 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.