I vaguely remember reading a question on main about a possibly "open question", which got an answer which started by affirming that the question is indeed open, and then proceeded to provide a solution to the open question, making it no longer open by the end of the answer.
I was specifically thinking about this because if I remember correctly it is an example of an open question being solved as a corollary of some (later) bigger results, and that no-one until the answerer noticed that the open question can be solved in that manner.
Unfortunately I remember none of the useful information that actually can help me search for the question/answer. In particular I know not what subject this was in, nor who the answerer was! Does the above description ring a bell for anyone?
:-)He linked to the relevant answer in the thread, while you linked to the question. $\endgroup$