6 September 2011: Switzerland caps the franc

The Swiss National Bank shocked markets by setting a minimum exchange rate of 1.20 francs per euro and pledging to buy unlimited foreign currency to defend it. Investors fleeing the euro crisis had pushed the franc to levels that hurt Swiss exporters. The cap held for more than three years, until the SNB abruptly abandoned it in January 2015, sending the franc soaring and wiping out traders who assumed it was permanent.

The lesson: Central bank pledges can end without warning, so never treat a currency peg as a guarantee.