It never occurred to her that if the drainpipes of a house are clogged, the rain may collect in pools on the roof; and she suspected no danger until suddenly she discovered a crack in the wall.

The image contrasts hidden, incremental causes with sudden, visible consequences. Clogged drainpipes (a small, neglected blockage) allow rainwater to accumulate out of sight until structural damage appears as a crack in the wall. Applied to a person—“her”—the line suggests a temperament that overlooks slow-building problems (emotional, financial, moral, or domestic) and only recognizes danger when it becomes undeniable. Flaubert often uses such concrete, material metaphors to expose self-deception and the fragility of bourgeois security: the household, meant to symbolize stability, becomes a system where minor negligence can quietly undermine everything.