Today’s Video: Black Sabbath – Changes
Close friends of the band, Led Zeppelin showed up in the studio during Sabbath Bloody Sabbath's recording. Drummer John Bonham wanted to play on "Sabbra Cadabra" but Sabbath wanted to play material other than their own for the occasion. In the end, the two bands had an improvised jam session which was recorded but never released.
Changes was not on the album, but a B-side to the title track’s single.--wiki