Pieta

This is the work that brought Michaelangelo into the public eye - he carved it at the age of 25. It's of the Virgin Mary cradling the body of Christ.

Apparently there's a story that he went to see it when it was displayed and that someone in the crowd had wrongly attributed it to another sculptor - which angered him, so he found a place to hide until night and then proceeded to carve on the sash that the Virgin Mary wears 'This was carved by Michaelangelo'.

Wonder if that counts as graffiti?