Only a few weeks ago, a number of emerging economies suffered currency crises. Argentina stands as an outstanding case that was covered in several media outlets. For instance, in a recent piece at Project Syndicate, Martin Guzman and Joseph Stiglitz argue that the crisis there was a surprise.
In economics, in particular in the study of business cycles, the claim of unexpected shocks has become almost an excuse to defend any policy carried out by government institutions. These unexpected events, sometimes referred to as black swans, by their nature cannot be predicted. Therefore there is nothing a government entity such as a central bank could have done to avoid the costly shock. Crises are not the result of bad policies, they are just a matter of bad luck. Good luck is to have policy makers in place to minimize the damage from these unexpected shocks.