The first paragraph of the Holmes story The Naval Treaty, from Memoirs of Sherlock Holmes, reads as follows (emphasis mine):
The July which immediately succeeded my marriage was made memorable by three cases of interest, in which I had the privilege of being associated with Sherlock Holmes and of studying his methods. I find them recorded in my notes under the headings of "The Adventure of the Second Stain", "The Adventure of the Naval Treaty", and "The Adventure of the Tired Captain". The first of these, however, deals with interest of such importance and implicates so many of the first families in the kingdom that for many years it will be impossible to make it public. No case, however, in which Holmes was engaged has ever illustrated the value of his analytical methods so clearly, or has impressed those who were associated with him so deeply. I still retain and almost verbatim report of the interview in which he demonstrated the true facts of the case to Monsieur Dubugue of the Paris police, and Fritz von Waldbaum, the well-known specialist of Dantzig, both of whom had wasted their energies upon what proved to be side issues. The new century will have come, however, before the story can be safely told. Meanwhile I pass on to the second in my list, which promised also at one time to be of national importance, and was marked by several incidents which give it a quite unique character.
Taken by itself, this seems like the kind of Noodle Incident reminiscence which is fairly common among the Holmes stories. On many occasions Watson as narrator refers to some adventure they had together which never made it to publication. Perhaps this is a way of reminding readers that Holmes had a great many more adventures than just those that were published.
But in this particular case, The Adventure of the Second Stain is in fact the name of a later story, from The Return of Sherlock Holmes. Its first paragraph reads as follows (emphasis mine):
I had intended "The Adventure of the Abbey Grange" to be the last of those exploits of my friend, Mr Sherlock Holmes, which I should ever communicate to the public. This resolution of mine was not due to any lack of material, since I have notes of many hundreds of cases to which I have never alluded, nor was it caused by any waning interest on the part of my readers in the singular personality and unique methods of this remarkable man. The real reason lay in the reluctance which Mr. Holmes has shown to the continued publication of his experiences. So long as he was in actual professional practice the records of his successes were of some practical value to him, but since he has definitely retired from London and betaken himself to study and bee-farming on the Sussex Downs, notoriety has become hateful to him, and he has peremptorily requested that his wishes in this matter should be strictly observed. It was only upon my representing to him that I had given a promise that "The Adventure of the Second Stain" should be published when the times were ripe, and pointing out to him that it is only appropriate that this long series of episodes should culminate in the most important international case which he has ever been called upon to handle, that I at last succeeded in obtaining his consent that a carefully guarded account of the incident should at last be laid before the public. If in telling the story I seem to be somewhat vague in certain details, the public will readily understand that there is an excellent reason for my reticence.
Did Conan Doyle always intend "The Adventure of the Second Stain" to be an actual story from the beginning, or was it originally just another Noodle Incident which he only later decided to write up as a story?
Question inspired by @Hamlet on Area51; I asked him if he was planning to post it here before doing so myself.