"Alas! An ill fate is on me this day, and all that I do goes amiss." So says Aragorn on the first page of The Two Towers (p. 403). He's lost Frodo, and while he's searching for the missing hobbit, he hears Boromir's horn and realize that other members of the Fellowship are under attack. On the next page, after seeing Boromir draw his last breath, Aragorn repeats his lament: "All that I have done today has gone amiss." The formulation is striking enough to stick in the reader's memory. Not "I have erred," or "I made the wrong decisions," but "all that I have done has gone amiss." Is Aragorn expressing himself this way to evade responsibility for bad choices, or to correctly identify a malign influence on his fortunes? Maybe both. At the end of the chapter (p. 409) he seems to recognize the roles of both his will and blind luck, when he says, "And now may I make a right choice, and change the evil fate of this unhappy day!"
Fate vs. free will, as an explanation for bad happenings. That may be what Tolkien's getting at with this wording. But I also think he's just trying to get us to remember this scene. Why? Because The Two Towers is such a bifurcated book. The story splits into two when Frodo and Sam take off on their own, but rather than intercut between their narrative and that of Aragorn, Gimli, and Legolas (as Peter Jackson does so masterfully in the films), Tolkien spends the first half of this second book following Aragorn and company exclusively. It's not until the second half that we rejoin Frodo and Sam, and then we follow them exclusively.
And when we rejoin Frodo, what do we find him saying, almost immediately (p. 590)? "All my choices have proved ill." This isn't happening precisely simultaneously with Aragorn's laments - a little time has passed by now since Frodo and Sam left Rauros - but it's close enough. The similarity in sentiment, if not wording - the idea that one's best intentions are being thwarted - reminds us of Aragorn, and that in turn serves to remind us that, in shifting to Frodo's story, we're traveling back in time several days. I imagine Tolkien realized that his decision not to intercut the two stories (after all, in the first half of the book he does some intercutting, switching between Aragorn and co. and Merry and Pippin) was bound to create an odd experience for the reader, and he did what he could to remind us, despite the time dislocation, that these storylines are parallel. This doubling of sentiment is part of that effort (along with Frodo and Sam seeing Nazgul fly overhead, Nazgul that Aragorn and co. will soon see).
The doubling of sentiment also subtly equates Aragorn and Frodo. When the company was whole, Frodo, though Ring-bearer, was a follower; first of Gandalf, then of Aragorn. Now Aragorn leads only two followers, while Frodo leads a party of his own, although he only has one follower. Furthermore, Aragorn has the right of leadership due to his royal lineage; Frodo only has the Ring, and his social status as Sam's superior. Nevertheless, he's now called upon to lead: his choices, all he does, now matter. If they go ill, amiss, there are serious consequences.