An Impossible Figure box which can be drawn but not built.
References
Fineman, M. The Nature of Visual Illusion. New York: Dover, p. 120-122, 1996.
Jablan, S. ``Are Impossible Figures Possible?''
http://members.tripod.com/~modularity/kulpa.htm.
Pappas, T. ``The Impossible Tribar.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 13, 1989.