Of all people to show up at a Foo Fighters concert: Michael Buble joins in

Foo Fighters surprised their fans by inviting pop icon Michael Bublé up onstage in San Francisco.

Foo Fighters regularly invite an audience member to sing Bublé's 2009 hit “Haven’t Met You Yet,” as part of a cover song medley. But Dave Grohl has quipped that those who volunteer, don't know all the words to the song.

He told the audience, "Every time someone says they know this song, they don't know the song."

Standing in the crowd last night was Buble himself holding a sign that said "I love Buble." So Grohl called on Bublé to come on stage.

Bublé flew in from Argentina to be there, because as Grohl explained… “There's no such thing as taking a joke too far."

By the way the Foo Fighters are one of the acts performing on our iHeartRadio Music Festival next month! (Buble won't be there.)

CAUTION: this video may be funny but also contains adult language