Here is a picture of the sheet music (Eine Klein Nachtmusik, movement 1). The odd accidental has a red freehand circle around it.
What does this natural sign mean? As you can see, the key is G Major and the C is left natural in G Major. There is no C sharp in the previous measures anywhere near (the last was in measure 54, in the previous section), so I doubt it's a grace note. This is also found in the fourth movement, measures 86. .
Again, here, there are no accidentals on B's or C's for quite a few measures. Any ideas what these accidentals could mean?
Don't want to spoil the fun, but this could be some sort of error (not saying it is, but it could be). This arrangement was dated May 2008 and I just found a revised version (Feb 2012) by the same guy. It drops the accidentals in both spots and corrects the measure numbers (in case you didn't notice they are off). This doesn't necessarily mean that it was a mistake, but it is a possibility. Either way, if you can make sense of it, go ahead and answer the original question. I just wanted to let you know that the problem is not necessarily you, but it could be the score. If the natural signs do make sense to you, go ahead and answer the question. :)