Space Barouche
The Space Barouche was a large...uh...petroglyph, for lack of a better term, rendered in spray paint on the ceiling of the 2nd SF Forum in the Hendrix basement. It depicted a crude rocketship with "B.18" mysteriously emblazoned on its side, a series of ellipses trailing behind in an abstract representation of thrust.
No amount of paint or plaster could efface the Space Barouche. The spray paint would always seep through, and it would become visible again.