How often do you see a Federal agent admitting perjury on videotape?
A while back, word leaked out of an ATFE training video in which the head of their National Firearms Act Branch (in charge of maintaining registrations of full auto firearms, etc.) had said the the registries had a high error rate, but not to worry, his Branch would always testify that they were 100% correct.
ATFE went to extraordinary lengths to keep the video secret. FOIA requests were denied, and when it came up in Brady motions it moved to seal the record. It apparently forgot to seal the record in one case, and the video got out. Here's a compressed MPEG-4 highlight tape (2.2 megs)
[UPDATE: On the linked page, the video is embedded. You have to wait for it to download (about a minute on hi-speed, or many minutes on dialup) and it should start on its own.]