Okay, so there's a scene in the movie where the family is having dinner. (The part where the Baroness complains about the missing candlesticks, i'm pretty sure!)

Shortly after Jacqueline talks about the prince letting go of the commoners, Margeurite waves her cup to be refilled.

Danielle does so, but she slips and drops the cup. Now, if i'm not mistaken, that cup is made of plastic! It sounds and looks as plastic cups would. I could be wrong, but i don't think that France had plastic cups back then. =]