You'll see a number of related responses by myself throughout these forums about overtreating and incorrect treatment. The practitioner can vary the intensity of the treatment through various needling techniques, strong or weak moxibustion, strong or weak cupping, strong or weak tuina, etc. In very general terms you can use stronger techniques on more robust people with excess conditions and you work more gently on weaker more deficient patients.
With regards to choosing incorrect points, I can say the following. Acupuncture is generally free from side effects but may also be relatively free of results if people are treated incorrectly. Certainly, consistent incorrect treatment may eventually cause or contribute to imbalances within a person and this should be avoided. One or two treatments that don't go entirely well can easily be accounted for in follow up treatments and is a part of the process overall - particularly in difficult cases.
As acupuncture is about promoting the bodies natural healing functions and removing obstructions to these functions, many of the points have broad usages. Because of this, acupuncture is much more forgiving than say herbal medicine. That said, you can waste an awful lot of your patients time not being clear in what you are trying to accomplish. Constructing and carefully recording effective point prescriptions is crucial to getting results. You may find the article - general point selection rules - helpful in this regard.
The only problems I have witnessed as a practitioner is when you mix strong intense techniques with incorrect needling. For the most part these intense techniques are not necessary to get results (my opinion), so they should only be used (again, my opinion) for local issues or isolated conditions where you are certain of the effect you want. This way you avoid any potential for problems.
In my early years I used to travel around from practitioner to practitioner getting treatments. This way I could feel and experience what they were doing and feel the incredible range of sensations from practitioner to practitioner. A very important part of my learning process initially but I paid for it a number of times. I've been overtreated and, quite honestly, outright irritated by some of these excessive techniques. As a practitioner you need to first be certain of your diagnosis and what you are trying to accomplish with the treatment. Then you need to question every step along the way - not just in what you are doing, but what you are doing that you might not -need- to be doing. Overtime you will generate more efficient treatment methods and avoid side effects and complications.