One person in Atheist thoughts on the shroud of turin? – Yahoo! Answers actually wrote:
[O]ne of the Christian defenses to the fact the Shroud of Turin does not carbon date properly is that up to 60% of the Shroud has been replaced over the years in order to make repairs to it. I’m not sure they understand that means it’s not only possible to reproduce the image on the Shroud through artificial means, but that the Church itself has copped to the fact they have happily painted over half the image back on.