In the past decade Federal Reserve officials have been flummoxed by a housing bubble that cratered the financial system, a long stretch of slow growth they failed to foresee and inflation persistently undershooting their goal. In response they engineered unpopular financial rescues, launched start-and-stop bond buying and delayed planned interest-rate boosts. “There are a lot of things that we thought we knew that haven’t turned out quite as we expected,” said Eric Rosengren, president of the Federal Reserve Bank of Boston. “The economy and financial markets are not as stable as we previously assumed.” In the 1990s, a period known in economics as the “Great Moderation,” it seemed the Fed could do no wrong. Policy makers and voters saw it as a machine, with buttons officials could push to heat or cool the economy as needed.