I'm surprised. I did this without any intention to be offensive. Yet I was told I was deeply offensive.
Long story short: @RobbieGoodwin posted an anecdotal comment under my question, saying that he has failed to show native Korean and Japanese people the difference between sounds L and R. He also said that specialists suggest foreign sounds may be impossible for adults to learn. I appreciated the comment and found the problem interesting.
Since the comment was an anecdotal one and did not provide citations, I felt that I can only "dig deeper" in this problem if I ask a separate question, which this problem mandates. I asked this question on Language Learning SE. I quoted @RobbieGoodwin's comment there. I felt that, while asking a question, I was supposed to explain the basis of my doubts ("share your research") and also that posting a valid anecdotal story can help attract better answers (since, I felt, the prevailing opinion on LLSE was that it was a myth that adults couldn't learn foreign languages well enough, I thought people would think twice before simply restating this opinion when faced with a valid story of grave difficulties in this regard).
I felt in right to quote @RobbieGoodwin's comment there since: (a) this comment, as everything on this site, is public anyway; I'd never publicly quote a PM; (b) I thought the quote added value to my LLSE question; (c) I've done something similar before and the author of the quoted comment hadn't taken offense; (d) I did this in good faith, not meaning be offensive in any way to @RobbieGoodwin, but only meaning to dig deeper into the issue.
Sadly, unfortunately, @RobbieGoodwin took offense. He said that what I had done had been an "offensive disgrace"; suggested to take the issue to the Moderation; asked me to, IIUC, delete my question on LLSE and promise the whole community to never do anything like that in the future.
Now I must say I'm kind of an... interpersonal relationships oaf. It's by far not the first time I'm doing something in good faith, only to see (to my surprise) other people very angry at me. For this reason I'm asking this question... to learn if I've indeed gravely run afoul of etiquette. I'm deeply sorry if anyone takes even more offense from this question – it is, again, not my intention to offend anyone – but I'm figuring that asking this question is the only way to learn if I've indeed done something wrong.
Of course, if it turns out that I indeed was offensive, I'm going to delete my question on LLSE.
Final note: I'm not sure if I should ask this question on English SE meta or LL SE meta. I chose this site, (though I'm not sure if this is the correct choice), because if I did anything wrong, I did so against English SE folks and not against LLSE folks; so if I am to apologize and ask if I have indeed wronged; I should do so before English SE and not LLSE.