The key transmission mechanism that turned a simple correction in the housing market into a global financial crisis were those bonds. Global banks had loaded up on these supposedly safe securities, and were at risk of becoming insolvent when their true value became known. Some banks blew up; others were bailed out. Either way the global credit system froze.

But even if you were clever enough in 2005 to see all of this coming, you wouldn’t necessarily have been able to cash in as successfully as the characters in ”The Big Short.” Figuring out exactly what securities to bet against — and how and when — mattered as much as the basic insight.

The movie captures this well, as the characters face a crisis of confidence when foreclosures begin to rise and their big bets against mortgage-backed securities aren’t yet paying off. (“I may have been early, but I’m not wrong,” says one character, the hedge fund manager Michael Burry as portrayed by Christian Bale. “It’s the same thing,” a skeptical investor retorts.)

Indeed, the movie has a glancing reference to how hard it is to translate the basic insight about mortgage securities into profits. We hear of a Morgan Stanley trader who had the insight that B-rated mortgage securities were at major risk, but believed that AA-rated securities would be fine, and so offset his bet against the former with a bet for the latter. It cost the firm billions.

One simple lesson of this is kind of obvious: It is hard to make a boatload of money trading financial assets, even if you are brilliant. But there’s a broader one, which explains why most everyone, including regulators (and journalists), was caught by surprise by the ferocity of the financial crisis.

A lot of people thought a decade ago that there might be a housing bubble. Few of them understood the connections between housing prices and poor lending practices; the connection from poor lending practices to complex, highly rated securities; the connection between those securities to the balance sheets of major banks; and the peril to the economy if just a few of them faltered.