It was a weird few hours! But it happened, and it turns out everything's still okay today. They didn't even play any Nirvana songs.
What they did do was play a song they've been working on together for months ("Cut Me Some Slack," which is fine, basically), because they're hugely successful career musicians who probably were eager for the chance to play and maybe do some creative work with other hugely successful career musicians. It's all pretty regular.
Also, Paul McCartney is such a hardened old pro (so is Dave Grohl, really). Did anyone earnestly think he was gonna use this opportunity to try his hand at "Rape Me?" I mean, I would very much like to see that, but I'm a terrible person, and I recognize that's not what would have been for the best here. They seemed like they were genuinely having fun, no one was injured, we can all go back to complaining about Kanye West's leather skirt, instead. Or thinking about more ways to raise money for hurricane relief. Either way.
Follow Virginia K. Smith on Twitter @vksmith.