The DVR talks to the TV over a HDCP cable. This is a new digital standard that carries both video and audio data and allows two-way communication between the set and the recorder. Part of this had the DVR authenticating that the set has the right encryption keys so that the bits aren't sent in the clear where they could be intercepted by someone wanting to "pirate" the material.
My DVR also has a S-Video output which I route over to my DVD recorder to record shows. It downsamples the content to standard video resolutions, but it doesn't downsample the original data, it does this processing on the frame buffer that's being sent across the HDCP connection.
I setup my recorder to dump out a 30 minute show, a Lisa Loeb performance that had run on HDNet. I started the record, checked the S-Video output, and everything looked great. A few days later, when I checked the DVD-R, I noticed that about three minutes into the dump, the screen had changed from the video to a "HDCP Copy Protection failure" message which persisted for the rest of the recording.
What happened? I turned off my TV. That prevented the TV from responding to the HDCP signal, so the DVR put up the failure message which then got sent over the S-Video connection to my recorder. Frown.
I can fix this in one of two ways: first, don't turn off my TV when I'm outputting programs. Second, use the "Copy to VCR" feature on the DVR to output the show, although that has other problems because you can't cancel it once it starts and you can't just output part of a program.