what flaw in the house might a careful observer find

Although the house and its decay are the first things one might notice when one approached the House of Usher, the eyelike windows seem almost human. Yet, they are not the most noticeable "flaw" in the house; there is a large crack along the side of the house, foreshadowing that the house is, indeed, coming apart. That crack ultimately begins the final "fall" when the family and the house fall apart.