Someone asked me yesterday how frequently people are injured doing yoga. It reminded me of these words from Thich Nhat Hanh:
Although this can be read to be about the river of feelings-emotions, it reminds me of the importance of working with pleasant, unpleasant, and neutral body-feelings as well. In my opinion, a key part of first learning to practice yoga is going mindfully and slowly enough that we can begin to distinguish for ourselves the difference between healthy and unhealthy; intense sensation and pain; and something unpleasant but not dangerous, as compared with something potentially harmful.
This process, like many aspects of yoga, is an ongoing and unfolding exploration. The inner teacher is the most important guide.