An Impossible Figure published by R. Penrose (1958). It also exists as a Tribox.
References
Draper, S. W. ``The Penrose Triangle and a Family of Related Figures.'' Perception 7, 283-296, 1978.
Fineman, M. The Nature of Visual Illusion. New York: Dover, p. 119, 1996.
Jablan, S. ``Set of Modular Elements `Space Tiles'.''
http://members.tripod.com/~modularity/space.htm.
Pappas, T. ``The Impossible Tribar.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 13, 1989.
Penrose, R. ``Impossible Objects: A Special Type of Visual Illusion.'' Brit. J. Psychology 49, 31-33, 1958.