What early details foreshadow the final crumbling of the house in "The Fall of the House of Usher"?
In the following passage, the narrator is describing the vault to which he and Usher move what they assume is the dead body of Lady Madeline:
It had been used, apparently, in remote feudal times, for the worst purposes of a donjon-keep, and, in later days, as a place of deposit for powder, or some other highly combustible substance...
This is an example of foreshadowing popularly known as "Chekhov's Gun," named after the famous Russian playwright. He said that if you place a loaded rifle on stage early on in the play, then it has to be used meaningfully later on, otherwise you're disappointing the audience's expectations.
And it's the same here. The detailed description of the vault and its contents foreshadows the enormous explosion that will take place at the end of the story, in which the House of Usher will be completely destroyed.
I fancied that I perceived, and for the first time, a full consciousness on the part of Usher, of the tottering of his lofty reason upon her throne. The verses, which were entitled “The Haunted Palace"...
The narrator recalls a poem recited to him by Usher. (A poem written by Poe and published separately called "The Haunted Palace.") The poem is positively laden with portentous doom, foreshadowing the House of Usher's imminent demise. The head is used as a conceit to stand for the palace. In that sense, a "haunted palace" expresses Usher's psychological deterioration. The use of the conceit in this way further illustrates the intimate connection between Roderick's mental state and the structural stability of the palace.
One of the most interesting is this one: the narrator notices a crack that runs a very long length in the estate's structure. This is symbolic as well as being an instance of foreshadowing. The long crack is symbolic of the fractured nature of Roderick Usher's mental capacities. He is in a deep depression and it only gets worse as the story goes on.
The crack is also symbolic of Roderick's relationship with his sister, Madeline. There is some evidence that their family had a history of incest, so it is not a stretch to say that perhaps Roderick and Madeline has this type of relationship. Neither was married and they lived with each other. They clearly had issues within their relationship and it was fragile at best.
The relationship between Roderick and Madeline crumbles, just as the house does, when Madeline is buried alive. She is able to make her way out of her tomb and falls upon a completely shocked Roderick, who dies instantly. The narrator is able to escape and turns to watch the house fall in on itself.