I was being driven crazy by the sound of my guitar. What’s wrong? The tuner says I’m fine. Am I going out of tune because I’m pressing too hard on the strings? Why do octaves all sound good? Is that a rattle/hum/buzz?

After 15 minutes, I realized what it was the interaction between the note I was playing and the sound of my ceiling fan being just slightly different such that I could hear the subtle beats of notes in unison out of tune. Turn off the fan, wait for it to stop. Play. Everything sounds fine.