The Knife!

The knife in Peter's hand is easy to miss if you aren't looking for it, but it's in plain sight. Why it's there, though, is not so plain. Neither art historians nor Catholics have solved this small mystery- largely because they don't see it as a mystery at all. Catholics claim it is simply a bread knife. However, readers of Dan Brown's The Da Vinci Code have come out in full force to give their opinions, noting that Peter appears to be hiding the dagger, and that he has a menacing look on his face. The most popular opinion is that it's Da Vinci's way of lashing out at the church. Some conspiracy-theorists even claim that the hand doesn't belong to any of the disciples- but in clear pictures it definitely appears to be in Peter's hand.