Cory picked up on PVRblog's coverage of what was eventually determined to be a bug: Users found their TiVos unexpectedly expiring recorded shows.
It might well have been a bug in this instance, but bugs like that don't just come from nowhere, with fully formed error messages alerting viewers that "Due to policy set by the copyright holder, 'Keep until I delete' is not permitted." Maybe it wasn't meant to show up here and now, on broadcast TV, but someplace in TiVo's corporate innards, someone decided that unrequested expiration was a feature.
Nothing in copyright law mandates this "feature." To the contrary, once you have a lawful copy of a copyrighted work, the first sale doctrine says you have the choice whether to save, lend, or discard it, while Betamax says timeshifting creates a lawful copy. If not copyright law, then copyright-holder muscle probably sits behind TiVo's design. Copyright holders work with Macrovision to implement extra-copyright controls, then jointly lean on TiVo to respond to them. Together, they restrict user rights beyond copyright.
The bug also illustrates the fallibility of proprietary technologies (particularly those with automatic update). "Update" doesn't always mean "improve" -- an update can take away functions you've come to enjoy, just because someone else objects. This misfeature of any DRM that implements "revocability" gives "planned obsolescence" a whole new meaning.
Like Cory, I've gone the MythTV route instead. With hundreds of people hacking on its open-source code, MythTV updates really are improvements. Its features are truly features, like commercial skip, time-stretch, transcoding and transfer to other media, plus an open-format music server on the side, giving full access to all the rights copyright reserves to the public. Sorry TiVo, you've been out-evolved.