From the 12.12.12. Sandy Relief Concert. [YouTube]

Great quote below from an article on The Guardian:

McCartney said: “I didn’t really know who they were. They are saying how good it is to be back together. I said: ‘Whoa? You guys haven’t played together for all that time?’

“And somebody whispered to me: ‘That’s Nirvana. You’re Kurt.’ I couldn’t believe it.” [Source]