Continuity mistake: At Molly's party after returning from Europe, the Duchess is introduced, and everyone bows. The shot shows Molly reaching for her neighbor who's standing straight, to bow down, and she does. Then there is a long shot showing the crowd, and the neighbor is again standing, and then you see Molly reach out for her again as she just had.
Factual error: At the start of the excommunication scene, the monks are chanting Dies Irae, a thirteenth century Catholic poem, but St. Thomas Becket died in 1170, almost a century before the poem was written.