I don't know whether this is a Logic issue, a Kontakt issue, or a problem with a specific virtual instrument. I have four instruments in one multioutput instance of Kontakt - two violins, a harpsichord, and a viola da gamba.
I've been doing a lot of detailed midi editing on this piece for several days now. Just now, while listening through for the nth time, the harpsichord (Sonicoture Renaissance collection) started sounding a semitone lower. The midi notes for the harpsichord still have the right names, but they sound a semitone lower. I assume that I must have accidentally touched something to make this happen but I don't know what it could be. I've checked the tuning for the instrument in Kontakt and that hasn't changed. This has never happened before.