The federal government's pressure on lenders Wednesday to fix the paperwork problems plaguing foreclosures left unaddressed a far greater potential threat facing the financial system and the U.S. economy.

Beyond sloppy documents, the foreclosure debacle has exposed one of Wall Street's little-known practices: For more than a decade, big lenders sold millions of mortgages around the globe at lightning speed without properly transferring the physical documents that prove who legally owned the loans.