Improper exercise (improper tai-chi included) can lead to a loosened joint. In particular, improper yoga recklessly aiming at flexibility can lead to rather severe joint loosening (check up the web or my recent post HERE).
What to do about it? Don't do any exercise? No way!
The correct approach is to do proper exercises to strengthen one's joints (BEFORE they have gone too bad!), and to do them properly. Doing zhan zhuang or tai-chi for joint strengthening needs to take the following exemplary precautions:
1. Do bend your knees to generate chi, but don't bend them too much! The "classical" advice for increasing one's exercise level is to bend more of your knees (i.e. ask your muscles to do more work). It is actually bad advice as far as joint protection is concerned. In addition, one should mindfully allow one's chi to flow and circulate around the knee joints to activate inner muscles to help stabilize any minor loosened areas.
2. Don't bend your wrists (i.e. palm and forearm should be at a straight line as much as possible) while doing tai-chi (like pushing out) if you're concerned with carpal tunnel syndrome. Again the "classical" advice is to do "push out wrist" (凸腕) as if pushing people out while doing the form. If one is concerned about strengthening one's wrist joints, one should do it like zhan zhuang instead, i.e. like fingers pushing out.
The above only act as examples, the gist of the matter is that it is a practitioner's responsibility to take care of his own joints (assuming that he chooses to do so in the first place), rather than his teacher's (or sifu's). And doing things in the "classic ways" might be a bad advice in some situations!