Fannie Mae’s recent disclosure that it made a $6.67 billion accounting classification error in its mortgage purchases raises fresh questions about the efficacy of its accounting practices, and throws fresh doubts onto the credibility of its efforts to clean up its massive accounting mis-statements. In its latest monthly business summary, Fannie disclosed that approximately $6.67 billion of single-family mortgage loans were incorrectly recorded as multifamily mortgages. To clean up this error, Fannie had to increase…