How many of us have noticed that one of the world’s most famous and moving paintings has a doorway cut right into it’s middle? Leonardo’s Last Supper covers one wall of a dining hall in the monastery, Santa Maria della Grazie in Milan. Apparently, in 1652, it was decided that the room needed another doorway, so one was created. It has since been walled up, but the intrusion into the painting remains.
For some insight into this painting, see this excellent video: http://smarthistory.khanacademy.org/leonardo-last-supper.html