Catherine completely missing the perfect opportunity to be a real heroine by assuming that the lady with Henry Tilney is his sister and fainting dead away in a very devastated and romantic manner, instead correctly taking her as Miss Tilney, Henry's sister.
John Thorpe asks Catherine to marry her without her realizing it, and her innocent answer is spot-on.
John Thorpe: Did you ever hear the old song 'Going to One Wedding Brings on Another?' I say, you will come to Belle's wedding, I hope. (...) And then you know, I say, then you know, we may try the truth of this same old song.
Catherine: May we? But I never sing.
Catherine's encounter with the cabinet. Explaining the sequence really wouldn't do it justice, as Austen's Affectionate Parody of breathlessly intense Gothic novel narration is essential to the humor.
The scenes where John Thorpe and James Morland brag about their horses and carriages.