Deliberate mistake: The main character is called Anthony Dorian in the pilot. But without explanation, his name becomes Anthony Blake in the first regular episode. (00:05:15)
Jessica's First Christmas - S2-E7
Deliberate mistake: Just before father O'Hara explains to Frank why he isn't in the nativity play he says to Betty "Perhaps I will have that cup of tea after all Mrs. Spencer" Betty goes to the kitchen but father O'Hara never gets his cup of tea.
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Suggested correction: Perhaps Betty just forgot.