Contrary to popular rumor, the "too small Stonehenge" disaster is not a parody of Black Sabbath's over-sized Stonehenge sets from the Born Again tour. The Stonehenge Spinal Tap scene existed as early as 1982, when the film was a 20-minute short. Black Sabbath didn't begin using their Stonehenge sets until 1983. The Stonehenge scene was likely inspired by the setup for Led Zeppelin's final two U.S. concerts, at the Oakland Coliseum, July 23 and 24, 1977. The stage was framed by a large, Stonehenge-like monolith. Additional Stonehenge models appeared on the stage and large banners to either side of the stage featured images of Stonehenge. The concerts are infamous because several members of Led Zeppelin's entourage beat up a local crew member backstage after the first day's performance.
I always thought the Black Sabbath thing was true.
Steven Tyler thought that it was inspired by the cover of Aerosmith's Rock In a Hard Place, and was offended by it at the time.