I often find it hard to explain the concept of off-key notes (called besur or kAnsur), because there are only 2 ways to explain it : using frequencies and using intuition/feel. Both usually fail.

If you want to give it a try, here’s one example:

Some second-order friends recorded this song. I liked the song, its tune and especially the female lead, but the guitar is off-key. If you have an untrained ear, see if you can spot it. Let me know how it went.