U2 was nearly done with their set at Philadelphia's JFK Stadium on September 25th, 1987 when a roadie brought out another microphone stand. "Anybody else want to play my guitar?" Bono asked the crowd. "Would Bruce Springsteen like to play my guitar?" As Bono literally bowed down before Springsteen, the crowd went insane. "I guess you guys know him," Bono said. "Is he a local boy or something?" They proceeded to play "Stand By Me" (though it would have been far cooler if he'd come out for the previous song: U2's groupie ode "Trash, Trampoline and the Party Girl").
Bono had dislocated his shoulder during a Washington, D.C. show five days earlier, and he brought fans onstage over the next few days to play the guitar. It's unclear exactly why Springsteen was at the show that day, but Little Steven & the Disciples of Soul were opening up. It's quite possible he was catching up with his once and future bandmate and got coaxed into coming onstage.
This collaboration was filmed by filmmaker Phil Joanou for his U2 documentary Rattle and Hum, but it wound up on the cutting room floor. Somehow or another, many hours of raw footage from the film fell into the hands of fans, circulating in collector circles for years. (Maybe some day an industrious fan will edit it into a better movie.)
In 2002, Bono returned the favor by joining Springsteen and the E Street Band for "Because the Night" at a Miami show. Three years later, Springsteen inducted U2 into the Rock and Roll Hall of Fame, duetting with them on "I Still Haven't Found What I'm Looking For." Springsteen went to MetLife Stadium in 2011 to check out U2's 360 tour, but he never made it onto the stage – though Bono did wrap up the show by reciting the lyrics to "Jungleland" as a tribute to the late Clarence Clemons.