I have some experience reading music, but today I came across a Bartok piece, "Melody with Accompaniment", part of Mikrokosmos Vol. 2. The key signature has one sharp, C#.
I was under the impression that one sharp always means the key of G, with the sharp note being F#. This piece has the tones C# and F, so why isn't it notated in the key of D, with a natural symbol prefixing every F?