Revealing mistake: When Mary of Guise is found dead by her nephew, he lies her down and puts his head on her chest. As he comes down, her eyes slap shut! If she were dead, they would remain open.
Revealing mistake: When the Three Musketeers are escaping into the river with Louis, Porthos throws another Musketeer into the stone wall. You can see the wall jiggle when he hits it.