The mask that is removed from Leo has 3 parts - a face mask which is held onto Leo's head, with a 2 piece hinged cage that locks in the back. In the flashback, where they show Leo being put in the mask, it is only 2 pieces. The face part is fused to the hinged cage section.
Close to the end of the film, when D'Artagnan is helping everyone escape through a back door of the Bastille, he says "Don't stop until you reach the river" and we see the door opening with all the characters in a particular order. I believe Athos is on the left of Phillippe. In the next shot, we see the same door opening from the outside and now the characters are lined up to exit in a completely different order. Athos is now on the right of Phillippe.
Just before the big battle scene, when everyone crosses swords on the ground, someone (I think Leo) is wearing white tennis shoes.
D'Artagnan's three best friends who have known him for over twenty years pronounce his name three different ways. Only the stupid one, Porthos (Gerard Depardieu) gets it right. "Athos" is also pronounced several different ways: either a hard or soft initial 'A,' and with the 'h' either pronounced or silent. Sometimes the name is pronounced different ways in different scenes by the same character.
After helping Phillipe escape, the mask is sent to Louis. But back at the house where Phillipe is preparing is revenge, one night he "actually tries the mask one last time," saying that he had it for so long, he's kind of used to it (or something to that effect). How can the mask be at two places at the same time?
6 years in a mask and he's able to keep that lovely babyface with just a little bit of grime, which easily comes off? Of course it would have wrecked the whole story but still...