In the practice of chi kung, the same side effect can be either positive or negative! A side effect that some literature talked about is "trapping of chi" in certain parts of a practitioner's body. The general advice is that it is bad for the body and should be avoided; and if it happens to a student, a qualified coach is to be consulted. Dropping out of the practice may end up to be the unavoidable final solution in severe cases. And, if the trapped chi is inside one's head, it might lead to severe headache or can even trigger an onset of psychosis! Doesn't it sound scary?!
Chi is powerful energy. When it is trapped in certain parts of our body, it will try to "reach out" and to "neutralize" itself. The target of reaching out is another chi (or excited) point. In the most generalized situation, it will try to reach out to our stretched hands (and fingers). For seasoned practitioners it will also reach out to their feet and stretched toes. For the most seasoned practitioners, it will reach out to any internal points energized by the practitioners themselves through subtle body adjustment, breathing and mental focusing. These internal points are essential for training. In tai chi chi-kung, these are our joints (that's why tai chi classics says: opening our nine joints), plus our Dantian. In meditation, they are our chakras. In microcosmic circulation, it is our spine.
Now for practitioners who are progressing along their learning path (which incidentally include the most seasoned practitioners), there will be cases, during chi movement, when such chi is blocked in certain parts of our body. With a path is blocked with chi still coming along the way, chi will be highly concentrated in one point (or small area), making the practitioner feel very uncomfortable. It can be quite scary when one experiences it the first time. This is the negative aspect of chi kung side effect. Some students will drop out when they encounter this.
Luckily there is a positive side. Trapped chi is really a challenge to a practitioner who aims high. With or without the help of his teacher, a courageous student/practitioner will try out methods (subtle movements and mental focusing) advised by his teacher. However such teacher-suggested solution is likely to be inadequate because a teacher does not share his student's same internal sensation (a good teacher of course can infer and deduce through observation and touching), and without feedback to any progress (with such progress being difficult to verbalize). Only with good feedback can a student further fine-tunes his solution accordingly. Therefore, in addition to his teacher's advice, a student will also need to try to improvise, and by trial and error, divert chi through the path that has been blocked, say frontal attack to clear the blockage or gently move chi sideways to bypass the blockage and soften it in the process.
His improvisation and trial and error methods, using his own body as his experimental subject, a student will be able to make big progress in his practice - both opening (or softening) the blockage and in the process understand his own body better. Courage and dedication to his art is essential. And only through such experience (which can be many, and will be getting more and more controllable/manageable), can a practitioner progresses to the highest level and become a true master of the art himself. Chi kung is a sophisticated and complex internal discipline. Only masters who have gone through such experiences (and benefited from them) who are qualified to teach.
The above comes from my own experience: learning and teaching.
(edited on 13 Dec 16)