The Father Christmas Affair - S4-E7
Revealing mistake: At the end, when Mr. Grace is inspecting the Father Christmases, you can see his secretary behind the scenery waiting for her cue.
The Father Christmas Affair - S4-E7
Visible crew/equipment: When Mr Humprhries is talking about the whereabouts of Mr Grainger, explaining about the inability to remove the Al Jolson make up, you can see John Inman looking up before having his face shadowed by a boom mike.