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 ...
This is an excerpt from Opus. 69 No. 2 by Chopin from the Henle Urtext: In the last bar seen in the excerpt, there is a sharp on the A in bass line. Since A is already "sharped" in the key ...