Beck joined the White Stripes on stage during the encore of their concert in Los Angeles. They performed 'Cold Brains' together, and then Beck played bass on the Stripes' 'Denial Twist'.
Beck reminisced about this appearence in Filter Magazine in 2010, "I played bass, which was fun. I felt a little unsure, you know, because they don't have bass in that band. Filling in with the bass was like taking something away-the alchemy or the magic of their music-but it was a great show." (I've seen some reports that said Beck played guitar on "Cold Brains," unsure which is correct.)
The reason for the reminiscence was because Beck attended this show with Nigel Godrich and Charlotte Gainsbourg. Nigel had tried to get Beck to work on Charlotte's first album, but it didn't work out. Beck and Charlotte didn't really meet until tonight, and this led to a few years later, they're collaborating on her second album.