Stonehenge scene
In the Stonehenge scene, Spinal Tap is completely shocked that their stage prop is tiny (in danger of being crushed by a dwarf!).
I know the movie is just a parody. But what's the chance that the band would be totally unaware that the Stonehenge rock was undersize until they were performing onstage? Usually a band would know what the stage set up would be like days or weeks before the show. Even if they were forced to use the mini Stonehenge, they would be aware of it!