Metallica and Lady Gaga's performance at the Grammys might have been one of the bigger let downs of the night, especially now that we have the dress rehearsal footage.
The actual grammy performance was marred by a significant technical issue when James Hetfield's mic went out for the first half of "Moth Into Flame," however even Lady Gaga's vocals differ greatly from the dress rehearsal, indicating that she may have been aware of the glitch and was also unable to recover.
If we had only seen the dress rehearsal maybe we wouldn't have found the performance to be so overall cringeworthy. You can still lose the backup "moshers" though.