As many rush to ease monetary policy even more to revive their lackluster economies, those that don't—such as the U.S. Federal Reserve and Bank of England—have found their growth penalized by rising currencies. That may contribute to a more accommodative policy path than they envisaged, reversing foreign-exchange rate gains and putting pressure on their counterparts to take further action.

This kind of vicious cycle may be one reason why investors are increasingly questioning the ability of monetary policy to spur economic growth. It marks a potential shift from what was supposed to be a year of divergence in central bank policies.

"If growth is disappointing and countries that hit the zero bound try to export their problems through currency weakness, that sucks everyone else closer to the zero bound," said Michael Feroli, chief U.S. economist at JPMorgan Chase & Co. in New York. "You have a contagion."