Potentially “thousands” of troubled homeowners were denied opportunities to lower their monthly mortgage payments under the Obama administration’s signature foreclosure-prevention plan due to servicer errors and inadequate oversight by the Treasury Department, a government audit has found.

Mortgage servicers failed to comply with basic guidelines, used different criteria to evaluate borrowers, recorded error rates up to six times their established thresholds, and couldn’t provide evidence that potentially eligible homeowners had been solicited for the administration’s Home Affordable Modification Program, also known as HAMP.

The errors are partly due to Treasury’s failure to issue specific guidelines for servicers to follow, and the administration’s lack of quality-control standards. Because servicers aren’t required to adhere to the same set of standards, there’s a risk that firms aren’t identifying practices “that may lead to inequitable treatment of borrowers or harm taxpayers through greater potential for fraud or waste,” according to a Thursday report by the Government Accountability Office.