A YouTube clip of Michelle Obama's DNC speech, embedded on BarackObama.com, was blocked due to a copyright complaint "from WMG, SME, Associated Press (AP), UMG, Dow Jones, New York Times Digital, The Harry Fox Agency, Inc. (HFA), Warner Chappell, UMPG Publishing and EMI Music Publishing." It's not clear what happened, though my money is on some combination of YouTube's copyright bots detecting the incidental background music from the convention; and several broadcasters uploading their own version of the footage and registering it as belonging to them with the YouTube copyright bots.
A YouTube spokesman downplayed the blockage: "After tonight's live stream ended, YouTube briefly showed an incorrect error message," he said via e-mail. " Neither the live stream nor any of the channel's videos were affected."
It's not clear what he meant by none of the channel's videos were affected as the video was unplayable.
The most likely culprit is YouTube's pre-emptive content filters, which allow large media companies to upload content they claim to own and automatically block videos that an algorithm decides matches their own. That would make the glitch the second livestream copyright-policing snafu in the span of a few days: On Sunday, a similar algorithm at uStream interrupted the livestream of the Hugo science fiction awards. The award show included clips of copyrighted videos, though the algorithm didn't know that the clips had been authorized.
In early August, an official NASA recording of the Mars landing was blocked hours after the successful landing, due to a rogue DMCA complaint by a news network.