Global rebound euphoria tests central bankers' nerves on economic risk

Central banks bear some responsibility for financial-market fervor after huge doses of stimulus and liquidity injections to keep economies afloat

With the world barely through the worst of an unprecedented crisis, central bankers are already wondering if the next one is around the corner.
 
From Washington to Frankfurt, what began months ago as a murmur of concern has morphed into a chorus as officials ask if a risk-taking binge across multiple asset markets might presage a destabilizing rout that could derail the global recovery.
 
Just last week, the European Central Bank and the Bank of Canada cited mounting threats, cognizant of the retrenchment that ensued during the 2008 financial crisis. Meanwhile Bitcoin’s dramatic swings after a warning about