You’ll be shocked to learn that a new banking rule might have negative unintended consequences, as described in the latest issue of Bloomberg Businessweek.

A rule designed to reduce risk in the financial system could end up punishing the wrong banks.

That’s what analysts, economists and some banks say will happen if regulators adopt guidelines proposed last year by the Financial Stability Board, which coordinates banking policy for the biggest economies. The rule, which would require banks to hold a minimum level of long-term debt that can be converted to equity if they fail, benefits lenders with big securities-trading businesses, the kind that led to the 2008 credit crisis.

The quirk is the result of risk weighting, the mathematical models that treat trading assets as safer than corporate or consumer lending. It will force banks such as Wells Fargo & Co. and Banco Bilbao Vizcaya Argentaria SA, which depend mostly on deposits for funding, to replace them with costlier bonds.

“The rule is distorted because it favors large trading banks,” said Alberto Gallo, head of European macro-credit research at Royal Bank of Scotland Group Plc. “The problem is using risk-weighted assets. Lending banks have higher RWAs, while trading banks have lower RWAs. The trading banks with a much more volatile business model will end up having half as much debt requirement as the more stable lending banks.”

One suspects that former BB&T and Cato Institute executive John Allison would not be surprised to learn that an effort to “do something” about a perceived problem in banking would yield perverse results.