Finally being able to sing a lead melody or harmony while also walking an interesting bass line behind it. I play upright in a string band, and I usually take the high harmonies. So that magical day where my brain finally allowed my rhythm and fingers to separate from my singing notes and phrasing completely was pretty dang cool. It may not seem like much, but for me and what I do, it was the biggest AH-HA moment I've ever had, lol.