|Will.i.am and Bono|
The Black Eyed Peas star has produced a whole host of high-profile artists' work but was incredibly nervous about being in the studio with the 'One' hit-makers.
He said: "I recorded for a month with U2. Even though I worked with Michael Jackson, their U2-ness intimidated me. I didn't know how to suggest a song from scratch.
"So I co-produced 'I'll Go Crazy If I Don't Go Crazy Tonight' and kept 'I Got A Feeling' for us."
The 'My Humps' hit-maker also spoke of an astonishing encounter with a fan who proved she knew his lyrics better than him.
He explained to NME magazine: "In Japan and this girl came up and says, 'I reckon in 'Boom Boom Pow' you say 'boom,' 101 times'. I was like, 'Yeah, right', but said, 'Thank you so much for listening to the song.'
"I got back on the train, counted my booms and she was right. We counted all the booms."