15 January 2015: The Swiss franc breaks loose

Without warning, the Swiss National Bank abandoned its cap on the franc against the euro, which it had defended for more than three years. The franc soared as much as 30% within minutes, Swiss stocks tumbled, and several currency brokers were wiped out by clients' losses. Just weeks earlier the central bank had called the cap a cornerstone of policy. Central banks can promise a lot until the promise gets too expensive.

The lesson: A central bank's commitment lasts exactly as long as it is willing to pay for it.