Apparently, Cauchy did not even forget or lose at least the papers of Galois. This is yet another example of how E.T. Bell and his Men of Mathematics, for all its literary virtues, are unreliable historical sources. See What resources are available for lives of recent mathematicians besides E.T. Bell's Men of Mathematics? for more on E.T. Bell and his writing style. As for Cauchy and Galois, Rothman has an informative chapter in his Science a la Mode, and the article it is based on is freely available online, courtesy of MAA: Genius and Biographers: The Fictionalization of Evariste Galois. Not only was it not Cauchy but Fourier who "lost" Galois's memoir (and he had a pretty good excuse), but the Galois-theory-on-the-eve-of-the-duel story is also a piece of Bell's fiction:
"We now encounter a major myth which evidently had its origin in the very first writings on Galois and which had been perpetuated by virtually all writiers since. This myth is the assertion that Cauchy either forgot or lost the papers (Dupuy, Bell) or intentionally threw them out (Infeld). Recently, however, Rene Taton has discovered a letter of Cauchy's in the Academy archives, which conclusively proves that he did not lose Galois' memoirs but had planned to present them to the Academy in January 1830. There is even some evidence that Cauchy encouraged Galois...
[...] This letter makes it clear that, six months after their receipt, Cauchy was still in possession of Galois's manuscripts, had read them and was very likely aware of their importance. At the following session on 25 January, however, Cauchy, while presenting his own memoir, did not present Galois's work. Taton hypothesizes that between January 18 and January 25, Cauchy persuaded Galois to combine his researches into a single memoir to be submitted for the Grand Prize in Mathematics, for which the deadline was March 1. Whether or not Cauchy actually made the suggestion cannot yet be proved, but in February Galois did submit such an entry to Fourier in his capacity of perpetual secretary of mathematics and physics for the Academy.
[...] The misfortune referred to above was the death of Fourier on May 16, 1830. Galois's entry could not be found among Fourier's papers and in Galois's eyes this could not be an accident. "The loss of my memoir is a very simple matter," he wrote. "It was with M. Fourier, who was supposed to have read it and, at the death of this savant, the memoir was lost." In addition to propagating the legend that Cauchy lost the manuscripts, Bell, curiously, does not mention Fourier by name in the preceding misadventure, although Dupuy is explicit on the identity of the Academy's Perpetual Secretary. Perhaps Bell felt it a little too much to "expose" Cauchy, Fourier and later Poisson as incompetents. Bell also does not make it clear that the papers listed above (plus a later memoir) constitute what is now called Galois theory. If this point had been clarified, the claim that Galois had written down the theory on the eve of the duel would be difficult to substantiate or even to suggest."
Rothman details Bell's other "creative contributions" to Galois's biography, like scrambled timeline, omitted events and manufactured quotes. Galois's early biographer, Dupuy, and Infeld share some of the "credit".
Taton's paper is in French, but it is not that recent (Rothman wrote back in 1982): Sur les Relations Scientifiques d'Augustin Cauchy et d'Evariste Galois, Revue d'Histoire des Sciences 24 (2):123-148 (1971)
