The bond market is enduring its worst selloff in a generation, triggered by high inflation and the aggressive interest-rate hikes that central banks are implementing. Falling bond prices, in turn, mean paper losses on the massive holdings that the Fed and others accumulated during their rescue efforts in recent years.

Rate hikes also involve central banks paying out more interest on the reserves that commercial banks park with them. That's tipped the Fed into operating losses, creating a hole that may ultimately require the Treasury Department to fill via debt sales. The UK Treasury is already preparing to make up a loss at the Bank of England.