no matter what your rank, is that you stop learning at that point. If you're going to keep growing, you have to be willing to be wrong. That's harder when you're in charge than when you're just another student.
That is so true! If I don't have space to be wrong, to totally screw up, even when I am teaching, I'm not going to progress much. Fortunately my students understand that. And as time goes by, it happens less and less. Every time I do a technique, even if I am explaining the basics of ikkyo for the zillionth time, I try to be mindful of my posture, timing, connection, etc. etc. That way I can learn something from everybody, even if my teacher is far away (which she usually is). I'm far from perfect, but that's what I strive for anyway.
As far as teaching goes, I started leading class when I was 2nd kyu, because there was nobody else. My wife taught occassionally at 4th kyu for the same reason. Generally I think that basics can be taught by folks at the shodan/nidan level.Regardless, Katherine is absolutely right - instructors need to get out, see their teachers, and train with their sempais in order to keep progressing.