I came across this notation in Schubert Impromptu No 2. Can someone tell me what it means?
♮#(note)
EDIT: I now believe this was just a printing error in the score.
Was there a flat on that note in the key signature, or in the measure before? If so, the natural might have been a kind of courtesy accidental. If the note was a D, for example. If there was a D flat in the key signature, the natural followed by the sharp would just make it clear that the note intended was D sharp, rather than D natural. This is obvious to us now, but there was a convention that was sometimes followed at least as late as the early Baroque that a sharp meant that you raised the note a semitone from what it was before. So that in C minor, a sharp on an A would raise it from A flat to A natural, rather than indicating that you should play an A sharp. Nobody uses that convention anymore, as far as I know, but in Schubert's time, enough people might have been familiar with it, that the natural sign followed by a sharp helped to clarify what was meant.