Now we know what ruined Metallica's Grammy performance with Lady Gaga. A stagehand apparently unplugged the wrong cable (and then probably lost his job).
TMZ cites a source close to the production who says the band and Gaga's line check right before the performance went fine. But a wire under the stage got unplugged by a stagehand by accident, leaving James Hetfield's mic dead. The glitch was discovered toward the end of the song, and by that point, Hetfield had already kicked his mic stand over in anger.
Metallica did refer to the incident at their show at the Hollywood Palladium Sunday night after the Grammys. At the end of the 10-song set Lars Ulrich walked up to the mic, joked about how nice it was that all the mics worked at the Palladium and suggested that the Staples Center borrow them.