Fannie Mae badly fumbled its accounting for MBS held in its portfolio but the company’s failures were caused by ineptitude – rather than deliberate attempts to massage its financial statements – according to an independent investigation launched by the government-sponsored enterprise. The massive report by former Sen. Warren Rudman released late last week concluded that Fannie Mae violated an “unambiguous accounting rule” on the classification of securities as either held-to-maturity, HTM, or available-for-sale, AFS. However,…