There is only one passage in the Gospels that was added hundreds of years later--it's John 7:53-8:12, the story of the woman caught in adultery (where Jesus says, "let he who is without sin cast the first stone"). However, even though this was added to the Gospel of John at a late date, the story itself dates back at least to 150 AD and probably earlier. (See Raymond Brown's book on John in the Anchor Bible Commentary for more details).