Other mistake: Langdon and Teabing make a point of how John the Apostle looks female in the painting The Last Supper, and claim that it shows that it was not John at all depicted by Mary Magdalene. During Da Vinci's time it was actually common to portray John with long hair and no beard as he was thought to have been quite young when he became an apostle. Furthermore, Da Vinci painted The Last Supper using a new type of paint, which unfortunately did not stand the test of time very well. After only a little more than half a century most of the color had faded and the painting had to be restored. Any female traits in John's face may be the result of whoever restored the painting, not Da Vinci himself.
Factual error: The large glass pyramid at the Louvre consists of 673 glass squares, not 666.