For Ed Silverman

Al Kooper improvised the organ playing during the studio sessions for "Like A Rolling Stone":

Al Kooper didn't know what to play. He'd told some half-truths to get into Bob Dylan's recording session -- the musicians were working on some song tentatively titled "Like A Rolling Stone" -- and Kooper had been assigned the Hammond organ. 

There was only one problem: Kooper didn't play the organ. He was a guitarist.

The first takes were predictably terrible -- Kooper was just trying not to get kicked out of the studio. But on take four, he suddenly found his chords. 

Kooper's playing was pure improv -- "I was like a little kid fumbling in the dark for a light switch," he would later remember -- but he ended up inventing one of the most famous organ riffs in modern music.

Source: http://scienceblogs.com/cortex/2009/05/creativity.php