How are overhangs shown on a topographic map?

An overhang is a rare exception to the rule that contour lines never cross. If one were to drill down through an overhang, the same X, Y coordinate would be crossed by two elevations. When this would occur, the contour line showing the lower elevation is drawn dashed (as opposed to solid).

http://geology.isu.edu/geostac/Field_Exercise/topomaps/topo_interp.htm

It would be a very rare occurrence except in very large scale mapping (and not common even then), as the overhang would have to be high enough to be noticeable in contour terms, and stick out enough to be noticeable at the scale of the map. A map for climbers giving localised mapping of an area would be the only case I could envisage. Overhangs tend not to be very far reaching, because stone is not a strong material under stresses other than compression (see 'Structures: Or why things don't fall down' by J.E. Gordon, Penguin Books). (Baloo has taught map reading to both adults and young people.)