Notices of unpaid taxes, unanticipated and little understood, will probably multiply as more people fall behind on their mortgages, said Ellen Harnick, senior policy counsel at the Center for Responsible Lending, a nonpartisan research and policy center in Durham, N.C.

Foreclosure is one way that beleaguered homeowners can fall into this tax trap. The other is when homeowners are forced to sell their homes for less than the value of the mortgage. If the lender forgives that difference, they are liable for income taxes on that amount.

The 1099 shortfall, as it is called, stems from an Internal Revenue Service policy that treats forgiven debt of all types as income even if the taxpayer has nothing tangible to show for it, unless the debt is canceled through bankruptcy.

Still more fallout from the sub-prime trap. Lost your house? That's okay, you only owe us $30,000 because you lost it.