The New Shelton Wet/Dry has a neat article on the Top 10 Radical Moments in Modern Art, for example:
1953. Robert Rauschenberg: Didn’t get it?
Rauschenberg bought a pencil drawing by one of America’s leading artists, Willem de Kooning. He then erased the drawing, signed it himself and exhibited the artwork as his own. By this act of vandalism, Rauschenberg spat on the sacredness of an original artwork. Specifically, Erased De Kooning established a historical precedent that the destruction of an artwork is important aesthetically.