Optical scan electronic voting machines employ software components that are customized for each specific election. Such software components are critical from a security and integrity point of view, as they define ballot layout and outcome reporting facilities. The possibility of these components to be tampered with represents a major concern as incorrect results may be produced due to either malicious or accidental corruption. Deviations resulting from corruptions can go unnoticed in the absence of testing and auditing and may not be detectable by election officials/poll workers using the pre-election testing procedures that rely on the machines themselves. This paper presents an actual auditing process for the AccuVote Optical Scan Voting Terminal (AV-OS) (manufactured by Premier Election Solutions) and the ensuing results from a recent statewide audit, showing that thorough auditing of a large sample of voting hardware, specifically the memory cards that contain custom software comp...