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.
Trivia: Set in 1924, a set of publicity shots of the Series 5 cast members, new and old, were rolled out, and one shot in particular was quite revealing. Sitting on the mantelpiece behind cast members Hugh Bonneville and Laura Carmichael, who play the Earl of Grantham and Lady Edith, was a modern-day plastic bottle of water.