This semester my guitar class is a group. Last night we were working on a three-part rhythm and my instrument went wildly out of tune. I left the room while the rest of the class continued but couldn't put the strings back in shape.
The thing had been working well up to then. I'd practiced for well over an hour before coming to class. And it seemed to be in tune when I'd started playing in class, but then wham! Or whang!?
Anyhoo... At first I couldn't figure out if the problem was with my tuner or my guitar. Because the tuner was reading the 2 string as the 4 string and the 4 string as the 6 string. I was confused. But I checked the tuner against my Stratocaster and realized the tuner wasn't the problem.
Long story short: I took the guitar back to the place I bought it in November. The proprietor listened sympathetically and tried to tune the thing. Ultimately, after a lot of discussion about temperature, humidity and acoustic guitars, he destrung it, massaged it with oils, rebored some holes and replaced a "plug thing" (can't remember what that cap is called that holds the string in the saddle) restrung it, retuned it and regaled me with some most excellent guitar playing.
I also bought a new tuner. But that's another story. The one I had is really more appropriate for electric guitars. This new one, which clips on, should be just right for the acoustic.
The new strings are really different. Kind of sexy. Smoother.