if you feel pain or back pain or whatever, it is a good instructor to teach you what not to do. or how to change things up.
for example, whenever I start feeling a little bit of knee pain, it is a sign that my hips are too tight, that I have not spent adequate time stretching or warming up my hips. Therefore, a small pain is better than a really really big catastrophic injury!
a tiny bit of knee pain is a good instructor, rather than a mega catastrophic injury in which you blow out your knees!