Barclays' Massive Loan Pool Eludes Notice

"Borrowing from central banks is supposed to be expensive when market functioning is normal. When market functioning isn't normal, these facilities look cheap and so it's perfectly reasonable for management in these circumstances to seek out cheaper funding," Reinhart says. He adds that the exact cost is difficult to calculate from the outside because prices can vary depending on the type of collateral banks use.

Barclays has had a run of positive news lately. It passed a stress test from its regulator last month, and shares have rebounded on news it is selling its iShares asset management business to private equity firm CVC Capital Partners for $4.4 billion. Its shares have far outpaced U.S.-based competitors like JPMorgan Chase(JPM) and Wells Fargo(WFC) in recent weeks, and have more than doubled in the past month.

While Barclays' health is not necessarily at issue, Reinhart says he worries the central banks' loosening of lending standards may prolong problems in the system as banks like Barclays postpone selling or at least marking down troubled assets.

"Rather than dealing with these impaired collateral, the ability to fund them at the central bank may allow management to delay the reckoning," he says.

Further, Tavakoli worries the central banks could be stuck with assets worth far less than they think. "It's kind of disturbing that the Fed and the ECB have decided to take on this kind of collateral, and they're really not equipped to scrub it," she said in a follow-up phone interview.

Collateralized debt obligations -- a broader category that includes CLOs and collateralized bond obligations -- are probably the best example of the so-called "toxic" securities at the heart of the current crisis. They have led to hundreds of billions in writedowns over the past year, wreaking havoc at once-proud institutions like Merrill Lynch, UBS (UBS) and Citigroup(C).