user-1: There is the much talked about example of "zen sickness".
But I wondered can any buddhist theory or practice can ever be harmful to a human?
I'm looking for an answer from anyone, not necessarily a Buddhist perspective. Though, of course, I'd be dismissive of anything unreasonable.

user8042: This is a simple tip for your mind. The real Buddhism is the religion which have mentioned by the Lord Buddha. Its the only way which shows the path to nirvana. If you want to study, feel and follow the Buddhism, then I prefer you to follow the real Buddhism which mentioned by the Lord Buddha. The real path(real Buddhism) will never let anyone to a harmful way. You may feel, if you able to find a little time to study the real Buddhism. Friend, I think, the "Zen sickness" that you have mentioned is a trend in modern culture.

user-1: i can say that Buddha was mindful of every word he said making sure they are beneficial to all beings.  However, there might be some thing that seems to be harmful in short term but actually good in long run.  For example, during Buddha's time, 500 monks committed suicide after contemplating unpleasantness of body and 5 skandha.  (Buddha often talked about the drawbacks of all things that create sensual pleasures).  Those monks took it over the top and felt disgusted by their conditions so they took the wrong way out.  Later, someone asked Buddha if he didn't see that coming, Buddha said those monks would have taken their lives anyway from past karma.  at least they got some true knowledge in dhamma while they were still here which would benefit them in the future. 
So in short,  practicing Buddhism according to Buddha (8 folds path) is not harmful.        

user295: 
But I wondered can any buddhist theory or practice can ever be harmful to a human?

Many people pratice certain techniques as Buddhist pratice. When you come across a pratice that someone says is a Buddhist pratice you should try to ascertain:

is this actually the Buddhist pratice comparing it with the original words of the Buddha in record or have you learned the right things
have you understood the pratice correctly and to the enough detail or do you understand the theory behind the pratice well enough
is what you are practicing the correct pratice or despite you have learned the right thing and know the theory are you putting it to practice the proper way
does your pratice fit your character type. Now within the the different meditation techniques there are suitability based on character type.

If any of the above are not met you cannot blame the pratice.
Also if you pratice Insight meditation then nothing can go wrong. More particularly you should contemplate on the arising and passing of sensations pertaining to:

Sensation from beauty or ugliness of physical form and sensations arising from the states of material form 
Sensation on their own right due to perceiving something is favorable or not
Sensation due to mental states
Sensations due to mental content

One case of Zen Sickness (Zen Sickness by zen master Hakuin) mentions about excessive striving. In Buddhism you effort should be balanced. Excessive striving can lead to craving, restlessness and doubt:

you are caving for the result or some experience which you are chasing after through meditation. You are accumulating craving instead of reducing it.
you are too frequently measuring your progress hence restless about getting the results fast or doubting the technique, the teacher, and your ability. Resentment follows if it is less and excessive pleasure if it is more. Also you are doing something other than meditation which is measuring progress. Also if measuring lead to doubt you are engrossed in doubting that practicing.


-1: <p>There is the much talked about example of "zen sickness".</p>

<p>But I wondered can any buddhist theory or practice can ever be harmful to a human?</p>

<p>I'm looking for an answer from anyone, not necessarily a Buddhist perspective. Though, of course, I'd be dismissive of anything unreasonable.</p>
 8042: <p>This is a simple tip for your mind. The real Buddhism is the religion which have mentioned by the Lord Buddha. Its the only way which shows the path to nirvana. If you want to study, feel and follow the Buddhism, then I prefer you to follow the real Buddhism which mentioned by the Lord Buddha. The real path(real Buddhism) will never let anyone to a harmful way. You may feel, if you able to find a little time to study the real Buddhism. Friend, I think, the "Zen sickness" that you have mentioned is a trend in modern culture.</p>
 -1: <p>i can say that Buddha was mindful of every word he said making sure they are beneficial to all beings.  However, there might be some thing that seems to be harmful in short term but actually good in long run.  For example, during Buddha's time, 500 monks committed suicide after contemplating unpleasantness of body and 5 skandha.  (Buddha often talked about the drawbacks of all things that create sensual pleasures).  Those monks took it over the top and felt disgusted by their conditions so they took the wrong way out.  Later, someone asked Buddha if he didn't see that coming, Buddha said those monks would have taken their lives anyway from past karma.  at least they got some true knowledge in dhamma while they were still here which would benefit them in the future. 
So in short,  practicing Buddhism according to Buddha (8 folds path) is not harmful.        </p>
 295: <blockquote>
  <p>But I wondered can any buddhist theory or practice can ever be harmful to a human?</p>
</blockquote>

<p>Many people pratice certain techniques as Buddhist pratice. When you come across a pratice that someone says is a Buddhist pratice you should try to ascertain:</p>

<ul>
<li>is this actually the Buddhist pratice comparing it with the original words of the Buddha in record or have you learned the right things</li>
<li>have you understood the pratice correctly and to the enough detail or do you understand the theory behind the pratice well enough</li>
<li>is what you are practicing the correct pratice or despite you have learned the right thing and know the theory are you putting it to practice the proper way</li>
<li>does your pratice fit your character type. Now within the the different meditation techniques there are suitability based on character type.</li>
</ul>

<p>If any of the above are not met you cannot blame the pratice.</p>

<p>Also if you pratice Insight meditation then nothing can go wrong. More particularly you should contemplate on the arising and passing of sensations pertaining to:</p>

<ul>
<li>Sensation from beauty or ugliness of physical form and sensations arising from the states of material form </li>
<li>Sensation on their own right due to perceiving something is favorable or not</li>
<li>Sensation due to mental states</li>
<li>Sensations due to mental content</li>
</ul>

<p>One case of Zen Sickness (<a href="https://buddhismnow.com/2015/09/12/zen-sickness-by-zen-master-hakuin/" rel="nofollow">Zen Sickness</a> by zen master Hakuin) mentions about excessive striving. In Buddhism you effort should be balanced. Excessive striving can lead to craving, restlessness and doubt:</p>

<ul>
<li>you are caving for the result or some experience which you are chasing after through meditation. You are accumulating craving instead of reducing it.</li>
<li>you are too frequently measuring your progress hence restless about getting the results fast or doubting the technique, the teacher, and your ability. Resentment follows if it is less and excessive pleasure if it is more. Also you are doing something other than meditation which is measuring progress. Also if measuring lead to doubt you are engrossed in doubting that practicing.</li>
</ul>
