Other mistake: When the dowager visits Matthew in his bedroom to tell him Mary is still in love with him before he goes through with marrying Lavinia, the curtains hanging behind Matthew are so wrinkled and creased just as curtains are when they come out of a package from the store. The crease lines are terrible! One would think set design would have steamed them as in real life, they would have been custom-made and certainly not folded up like that for shipping, thus never having creases like that.
Continuity mistake: Mary has written a letter to Matthew. We hear her voice reading the text, which begins: "So there we have it. I look forward to introducing the two of you, just as soon as you are next at home..." Next, when the page of the letter is shown, it reads "...happy to hear - come to pass. I look forward to introducing you to Sir Richard, just as soon as you are next at home." (00:11:03)