Everyone knows the story of how Roy Harper was just basically hanging around the studio and ended up singing lead on one of Pink Floyd's best tracks. I recently learned that Brian Jones played vibraslap on "All Along the Watchtower". I already knew that Dave Mason played the 12 string acoustic guitar but it got me thinking....what an interesting band that would have been...Jimi Hendrix, Brian Jones, Dave Mason, Mitch Mitchell ! Can anyone think of any more of these little "one time" super groups that made a great song? This list should not include paid studio musicians such as Hal Blaine...we all know he saved the day on hundreds of tracks. Looking more for the guys that were probably just hanging out or were casually asked to record on a track.