At the end of Don DeLillo's Falling Man, Keith, the protagonist, finds himself back where he was at the beginning: wandering the streets dazed and wounded as the Twin Towers fall. The last thing he sees amidst the chaos is a shirt falling from the sky, "arms waving like nothing in this life." What he has seen, we surmise, is not merely a shirt—it is a falling man—but that reality is too shocking. It's easier to consider it an inanimate object.
Keith has seen the falling man before. This detail, like so many other aspects of the novel, brings him full circle, back to the beginning. The impossibility of escape from the events of September 11th, which is emphasized by this circular structure, is further reinforced by the reappearance of the falling man, who is like nothing in most people's lives but who has already appeared in this particular life, underlining the strangeness of Keith's circumstances.