In Section 1, as Mary and Edward debate what to do with the perplexing arrival of a large sack of gold, Mary reveals serious misgivings about Hadleyburg. Her concerns that the town is, despite its reputation, a place of considerable vice and vanity foreshadows the revelations of the rest of the story:
It is a mean town, a hard, stingy town, and hasn’t a virtue in the world but this honesty it is so celebrated for and so conceited about; and so help me, I do believe that if ever the day comes that its honesty falls under great temptation, its grand reputation will go to ruin like a house of cards.
As Mary asserts, if it weren't for the supposed honesty of Hadleyburg's villagers, the town would be regarded as a morally bankrupt place. Her hypothesis that a "great temptation" could cause this honesty to crumble "like a house of cards" predicts the exact events that transpire after the arrival of the gold: the temptation that the gold presents causes Hadleyburg's prominent citizens to each falsify claims to it, thereby revealing just how conditional their honesty must be.
This passage ties directly to the central theme of vanity and virtue that Twain explores throughout his story: the citizens' honesty, rather than being a virtue, is the source of their extraordinary vanity—or "conceit," in Mary's foreshadowing words.
