An Italian scholar, Slavisa Pesci, claims to have uncovered new secret images hidden in Da Vinci's Last Supper by superimposing the painting on to its mirror image. When you do this you can supposedly see a woman holding a child, as well as a goblet in front of Jesus. Personally, I can't see anything at all. It all looks like a blurry mess. But Pesci seems to feel that Da Vinci intended for his painting to be viewed this way. He says, "from some of the details you can infer that we are not talking about chance but about a precise calculation."
Two years ago I posted about another Last Supper theory: The idea that an image of the holy grail is hidden in the painting, located on the wall above the head of St. Bartholomew, the disciple at the extreme left.
The hidden holy grail theory seems a lot more plausible to me than this mirror-image theory. (Thanks, Big Gary)