As a self-employed professional and homeowner, I understand the usefulness of "stated-income" mortgage lending. But I agree that taking the borrower's word for something as fundamental as his ability to repay is a completely inappropriate basis upon which to make a rational lending decision. The secondary mortgage market could be characterized, therefore, as irrational. The question begged, then, is: to what extent?

I don't sense a lot of transparency in the mortgage-backed securities market. Maybe your other readers can shed some light on the extent to which these loan portfolios contain what we used to call in banking "classified" loans; i.e. loans posing actual or potential loss exposure.

Last week, H&R Block, the big tax preparer, alerted Wall Street that its Option One Mortgage unit, which focuses on the subprime market, would have to set aside about $60 million, or 19 cents a share, because borrowers were falling behind on their payments.

Here's what's been happening: Mortgage originators make loans and then sell them to investment banks, which mash them together with other loans and slice them up like sopressata for sale to institutional investors.

Now, with the mortgage market slowing and the secondary market for mortgage-related securities faring modestly worse than in the past, investment banks are scrutinizing the loans that come into their sausage factories more carefully.

The investment banks have been sending mortgages back to the lenders if they find slip-ups, such as inaccurate paperwork or poor performance. The most common trigger is a so-called early payment default, where the mortgage holder has missed the deadline for the first payment.

Lenders complain that the investment banks are taking advantage of a contractual loophole to push the mortgages back. Customers often miss first payments, they say, for reasons that have nothing to do with credit worthiness. Sometimes it's just an indication of an administrative delay.

But that's probably wishful thinking.

If the problems spread beyond customers with poor credit, they'll infect the world of exotic mortgages for supposedly credit-worthy customers first. Along with the companies that offered mortgages to customers who didn't produce much documentation of their income and assets, the more vulnerable will be banks that sold huge numbers of option adjustable-rate mortgages.

Option ARMs give borrowers choices to minimize their mortgage payments, including the ability to make a minimum payment that is lower than the interest due that month. When a customer chooses that option, the mortgage balance goes up. After a certain period, often just a year, the rate can move up sharply.

Skeptics have wondered whether customers understood the full costs of these loans and whether lenders correctly estimated how likely it was they'd be paid back.

In an indication that there was reason to worry, Washington Mutual, one of the country's biggest mortgage lenders and a big option ARM player, slipped in a rather stunning confession in its annual filing with the Securities and Exchange Commission.

In the filing, WaMu confessed it had bungled the underwriting for option ARMs, improperly measuring some of its customer's debt-to-income ratios for 2004 and most of 2005.

Of $43 billion of such loans, WaMu discloses, the unpaid balance for borrowers who were qualified at below the market rates totaled $30 billion.

Option ARMs have been among the most scrutinized exotic mortgage products over the past year. That such an error could creep into WaMu's lending should worry investors not only about the bank's balance sheet but the industry's lending standards as a whole.

Option ARMs didn't go to subprime customers. That's precisely why the coming mortgage problems may not be isolated to customers with poor credit. (emphasis added)