We have this term literally translated as 'an other-way-round teacher' =
a bad teacher who after all proved to be a good teacher
because some of the students have thought "I will never become such a person"
This relationship should not necessarily be a teacher-student one.
It can be parent-child, boss-subordinate, older brother-younger brother, etc.
Is there any such word in English?

Thank you.
A bad example.

'A dark horse' refers to someone who surprises you , usually in a way that is not unpleasant.

Best wishes, Clive
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Thank you very much, everybody.
a bad teacher who after all proved to be a good teacher
because some of the students have thought "I will never become such a person"
I believe your focus is that the teacher was "good" only insofar as he provided an example of what not to do if you wanted to be a good teacher -- not that the teacher was ever really a good teacher or ever really came to be seen as a good teacher.

Furthermore, you want an adjectival form that expresses this example of what not to do.

That's a tough assignment. (only) by negative example comes to mind (The teacher was good only by [providing a] negative example.), but is unsatisfactory in many ways. I don't think there is such a word in English. The idea may have to be expressed in a different way. cautionary tale is another phrase that comes to mind, but a simple substitution in the form *a cautionary tale teacher absolutely will not work. object lesson is similar in meaning, but has the same problem. You'd have to say something like His teaching technique was a [cautionary tale / object lesson] in bad teaching.

In the final analysis, if I had to choose, I would translate an other-way-round [teacher / parent / boss / ...] as a perfect example of a bad [teacher / parent / boss / ...]. For a bad you might substitute an incompetent.

He was an inspiration to his students (only) in being a perfect example of [a bad / an incompetent ] teacher.

Thank you very much for the valuable explanation, Calf Jim.
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Jim, don't you think that "a bad example" serves that purpose?
How would you use it as a substitute for 'other-way-round' in the adjectival meaning described?

an 'other-way-round' teacher = a bad example of a teacher?

Yes, I suppose that would serve the purpose, although it's awfully close to saying 'other-way-round' = 'bad':

an 'other-way-round' teacher = a bad teacher.

I don't think any of us has come up with the exact shade of meaning the original poster seemed to be describing. I was trying to get the idea of 'good' into it with 'perfect' (I could have said 'excellent') example of a bad (teacher) - to show that the teacher was actually a 'good' teacher (for 'other-way-round' reasons). As I said, I'm not sure there is a way to say it in English -- nothing succinct anyway.

Or else I'm just reading more into it than I should be! Emotion: smile

As I said in my original post, this person may not necessarily be a school teacher.
Our term in Japanese is HANMEN KYOSHI. I don't know if it exactly translates
into 'an other-way-round teacher'.
Suppose I hit my wife all the time. My son sees it. He finds it a terrible thing to do.
He is determined not to do such a thing when he gets married. Now that he is
married, he is an ideal husband. In that case, I have served as a HANMEN KYOSHI
for my son, although I didn't intend to be one.
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