Halfway through U2's set Wednesday night at Madison Square Garden, lead singer Bono reached into the crowd for a volunteer to help him sing the next song. He picked a guy holding up a sign that said, "singer with a broken finger."

Once chosen, the man lowered the sign to reveal he was none other than "Tonight Show" host Jimmy Fallon, in a cast after falling in his home last month. Saying he was "so lucky and honored to be . . . in the greatest venue in the world with the greatest band in the world in the greatest city in the world," Fallon got a standing ovation.

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He went on to sing "Desire," donning Bono's glasses to impersonate the Irish rocker halfway through the song. Collapsing at the end, Fallon admitted, "It's not easy to be Bono!"

Fallon followed up with another surprise, telling the crowd: "I brought a few friends of mine . . . you guys know 'em, from Philadelphia -- The Roots." Five members of his late-night band then joined U2 for a rendition of "Angel of Harlem."

With a smile on his face, Bono told the crowd, "A little surprise at Madison Square Garden."