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A surprise departure from the European Central Bank could highlight the treacherous road ahead for the world's monetary policymakers.

It's not clear exactly why Sabine Lautenschläger, Germany's representative on the central bank's executive board, recently opted to resign. (Her term would have run until 2022.)

But the announcement that she was stepping down followed the central bank's high-profile decision to push interest rates further into negative territory and to restart its bond-buying program, a policy Lautenschläger has publicly criticized.

One takeaway: In a world of sluggish growth, stubbornly low inflation and fears of an economic downturn, there may be more room for doves than for hawks. Markets and companies are hooked on cheap borrowing. Meanwhile, central banks' ability to guide stable inflation and provide efficient stimulus is increasingly in question.

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