He looked as handsome and as lively as ever, and was talking with interest to a fashionable and pleasant-looking young woman, who leant on his arm, and whom Catherine immediately guessed to be his sister; thus unthinkingly throwing away a fair opportunity of considering him lost for her for ever, by being married already. But guided only by what was simple and probable, it had never entered her head that Mr. Tilney could be married. (8.3)
Catherine actually avoids folly here, by using common sense to not make a faulty assumption. While Catherine makes a lot of foolish assumptions, it's important to note that she does have common sense to draw upon.
Catherine's expectations of pleasure from her visit in Milsom-street were so very high, that disappointment was inevitable; and accordingly [...] she found, on her return, without spending many hours in the examination of her feelings, that she had gone to her appointment preparing for happiness which it had not afforded. (16.1)
Overly high or foolish expectations are a running theme throughout this text. Catherine here sets herself up for a fall by having overly high, and romantic, expectations for her visit to the Tilneys' house in Bath.
"Nay, my sweetest Catherine, this is being quite absurd! Modesty, and all that, is very well in its way, but really a little common honesty is sometimes quite as becoming. I have no idea of being so overstrained! It is fishing for compliments. His attentions were such as a child must have noticed [....] So it is in vain to affect ignorance." (18.11)
This speech is highly ironic coming from the often phony Isabella. She is berating, or scolding, Catherine for the folly of ignorance, or pretended ignorance, regarding John's "obvious" marriage proposal.
She felt humbled to the dust. Could not the adventure of the chest have taught her wisdom? A corner of it catching her eye as she lay, seemed to rise up in judgment against her. Nothing could now be clearer than the absurdity of her recent fancies. To suppose that a manuscript of many generations back could have remained undiscovered in a room such as that, so modern, so habitable! (22.2)
Catherine makes a foolish assumption here, regarding the "manuscript" that turns out to be a laundry list. Catherine's imagination has run amok, a motif in this book, and led her into folly.
Catherine's interest in the deceased Mrs. Tilney augmented with every question, whether answered or not. Of her unhappiness in marriage, she felt persuaded. The General certainly had been an unkind husband. He did not love her walk: - could he therefore have loved her? (22.37)
Catherine's folly is two-fold here. First, she makes a disastrous assumption about General Tilney's character based on really flimsy evidence. Secondly, Catherine's wild imagination leads her to behave thoughtlessly to Eleanor. Instead of sympathetically talking with Eleanor about her mom, Catherine interrogates her, hunting for juicy details.
Something was certainly to be concealed; her fancy, though it had trespassed lately once or twice, could not mislead her here. (23.6)
Catherine's faulty confidence in the power of her imagination is highlighted here.
Catherine's blood ran cold with the horrid suggestions which naturally sprang from these words. Could it be possible? - Could Henry's father? - And yet how many were the examples to justify even the blackest suspicions! (23.13)
The "examples" Catherine draws from here are Gothic novels, which she uses to indict General Tilney for murder. Catherine foolishly applies fiction to the real world, using Gothic literature as solid evidence.
Astonishment and doubt first seized them; and a shortly succeeding ray of common sense added some bitter emotions of shame. She could not be mistaken as to the room; but how grossly mistaken in everything else! - in Miss Tilney's meaning, in her own calculation! [....] She was sick of exploring, and desired but to be safe in her own room, with her own heart only privy to its folly [....] (24.6)
Upon seeing Mrs. Tilney's rooms for herself, Catherine realizes the extent of her folly. Catherine is embarrassed and ashamed by her realization.
"Miss Morland, consider the dreadful nature of the suspicions you have entertained. What have you been judging from? Remember the country and the age in which we live. Remember that we are English, that we are Christians. Consult your own understanding, your own sense of the probable, your own observation of what is passing around you - Does our education prepare us for such atrocities?" (24.29)
Henry delivers a climactic address to Catherine here, scolding her for letting her imagination get out of control and encouraging her to use common sense. It's interesting that Henry draws upon patriotic pride – the idea that the English are civilized and "reasonable" – to get through to Catherine.
The visions of romance were over. Catherine was completely awakened. Henry's address, short as it had been, had more thoroughly opened her eyes to the extravagance of her late fancies than all their several disappointments had done. Most grievously was she humbled. Most bitterly did she cry [....] Her folly, which now seemed even criminal, was all exposed to him. (25.1)
The narrative style shifts here to emphasize Catherine's distress. Shorter and blunter sentences highlight Catherine's agitated emotions. Notably, Catherine required some sort of intervention, in this case from Henry, to "wake" her up from her foolish delusions.
There were still some subjects indeed, under which she believed they must always tremble; - the mention of a chest or a cabinet for instance [...] but even she could allow, that an occasional memento of past folly, however painful, might not be without use. (25.4)
Catherine notes that memory plays a large role in preventing folly, and that forgetfulness often allows for folly. As such, Catherine accepts the need for having the occasional "reminder" of past transgressions, or mistakes, around her to keep her imagination in check.