It's the model, stupid!
“The essential problem is that our models – both risk models and econometric models – as complex as they have become, are still too simple to capture the full array of governing variables that drive global economic reality.[...] But risk management can never reach perfection. It will eventually fail and a disturbing reality will be laid bare, prompting an unexpected and sharp discontinuous response..” Alan Greenspan, former Governor of the US Federal Reserve, writing in the "Opinion" column of the FTMarch 16 2008.