I was playing a really nice Gibson acoustic at a benefit, supplied by them. I brought my own headstock tuner. I played and sang a few songs by myself, it was ok but something felt weird. I had asked my friend a very nice female piano player to join me on the last few songs. I started into Copperhead Road in D, she came in and was out of tune. I gave her a bit of a look and she adjusted very quickly. The next day i was watching a video somebody took of me, I picked up my guitar, and sure enough, I was playing a semitone sharp. I'd had the tuner for a while, it always worked fine, and I hadn't accidentally changed the setting, it just decided I wanted E# all of a sudden. I apologized to my friend the pianist the next day.