Spinal tap story, based around alternative band The Fall.
The Fall's eccentric leader Mark E Smith used to play Spinal Tap movie a lot on the tour bus, which meant the rest of the band were forced to watch.
One night bassist Steve Hanley played the main riff to a Spinal Tap song as a soundcheck. Mark E Smith said something like "That's good, save it for the next album". Steve assumed he was joking, and really recognised the riff as Spinal Tap.
However, come the time to make the album Smith insisted that the riff was used as the basis of a song to the consternation of the rest of the band.
(Spinal Tap "Tonight I'm Gonna Rock You Tonight" > The Fall "Athlete Cured")