NFA Record Problems
The National Firearms Act of 1934 requires a tax payment, and thus registration, for each making or transfer of certain firearms (chiefly full-automatic, or short barreled guns). Possession of such a firearm without registration is a felony, and in any prosecution for that offense an element is certification by ATFE that the firearm is not registered to the defendant. ATFE is in charge of maintaining the NFA database, which has several hundred thousand firearms listed.
As far back as 1980, it was known that the database had serious flaws and omissions (an ATF internal memo of the period expressed fears that innocent persons might be convicted due to a certification that a firearm had not been registered, when in fact it had been).
Eric Larson has over many years done yeoman work in further investigating the problems and in pushing for Inspector General investigation of the problem (the IG in turn told ATFE to look into it -- although the entire reason for having an IG is to deal with problems where the agency itself might have a temptation to cover up.