Even geniuses make mistakes. Consider Leonardo da Vinci and his 500-year-old masterpiece "The Last Supper". The mural was commissioned in about 1495 for a monks' dining hall in Milan. Your average Renaissance fresco artist would have squared off the wall, slapped on some wet plaster, and waved his magic brush. Speed was essential to ensure that the sections of paint and plaster dried together, making the picture part of the wall.
But Leonardo did not work like that. Some days he never put his paintbrush down; other days he never picked it up. (Not to mention the many other days he spent walking the streets looking for suitable male models.) He needed flexibility and to be able to paint over the same surface more than once, so he chose a technique usually used on canvas or wood.
He prepared a base of plaster, pitch and mastic gum and spread it on the wall. Then he settled down to spend the next four years working on what many regard as the world's finest painting.
But his oil and tempera mix never really melded with the dry plaster base and the room's damp atmosphere made the problem much worse. Soon the colours started fading, spotting and even falling off. By 1556 "The Last Supper" was said to have become "a muddle of blots".
Eventually, inevitably, the restorers got involved. The first recorded attempts were in the 18th century. One chap covered the mural with oil and varnish. The next one removed it, but then repainted much of the original in oil. Yet another tried to remove it from the wall. And so it went on.
There were nine disastrous attempts at restoration, each one leading to yet more repainting. The latest controversial restoration was completed in 1999. It took 20 years to strip the painting back to what was left of da Vinci's original - about half the huge piece. Any holes were touched up with watercolours. For the first time, say the restorers, nothing has been added. So let's hope that's it for a while. After all, Jesus was supposed to be betrayed by Judas - not the art establishment.