Like the other threads, but about musicians.
I'm assuming these people are musicians. I'm pretty sure they are. I just find it really annoying when someone in the audience points out that a performers on stage is playing out of tune. This person always sits right behind me. I hear the sour note on stage. Then I hear Mr./Ms. Know-it-all behind me say to their friend, "Sharp/Flat," with a condescending tone in their voice.
I have heard musician do some of those things on the other lists too. This tuning one though, man. If it's sour, don't say anything. We know. And if we don't, don't tell us.