The past few years have seen central bankers invested with almost superhero powers. "Super" Mario Draghi might not be one of the heroes in "Avengers: Age of Ultron" (although may be there would be room for him in a further sequel?), but his powers are almost as impressive as those of key character Iron Man.
And has Iron Man ever pumped so much money into a financial system that he kick-started a moribund region like the euro zone? I bet he hasn't—although Iron Man's arguably a snappier dresser than Draghi, much as we love seeing what color tie the central banker wears to the European Central Bank's monthly press conference.
With the bond markets now firmly in comic-book territory, Poland last week became the first emerging market to sell its debt at a negative yield. I in no way want to denigrate Poland, which has shown robust success in its economy in the past few years, but it is still six investment notches (according to Standard & Poor's) below neighboring Germany for good reasons. It is only in extraordinary times, with negative interest rates and investors desperate for yield, that it would be selling debt at -0.2 percent.
So when and how can central banks get back to normal life? As there is no precedent in economic history for negative nominal interest rates, it's difficult to predict. After all, Iron Man's alter-ego Tony Stark has a lot of difficulty staying out of the Iron Man suit when asked to do so.
The Bank for International Settlements, one of the few financial institutions to see the last crisis coming, has already warned of the potential for "significant and widespread losses on investors, with potentially serious consequences for financial and economic stability," as the lines between risky and less risky assets becomes blurred.
There are also concerns that destructive currency wars may break out, if countries and currency zones all try to depreciate their currencies at the same time.
With quantitative easing, there has always been a danger that central banks are fixing yesterday's problems. Using looser monetary policy to balance tighter fiscal policy might have helped to prevent the last crisis. The next one is unlikely to take exactly the same shape.
The flood of money and asset-buying may allow governments to put off reforms, by giving politicians an excuse to procrastinate when economic crisis seems far away.
When the exhilaration of flying in a fantastical suit wears off, investors could be coming back to Earth with a bump.