I wrote a comment under an answer, claiming that the answer was wrong. The poster replied to me in another comment, showing that my argument was wrong. Acknowledging this, I deleted my comment and then flagged the poster's one as "No longer needed". To my surprise, the flagged comment disappeared immediately, and my flag got marked as "useful", seemingly without any moderator acting upon it (or the author of said comment). Why did this happen? (If it matters, the question, and therefore the answers, were all "community posts".)

Edit 01.01.2018: Following some suggestions in the comments, I asked the poster whether he deleted his comment himself right after I had flagged it. No, he didn't, so the oddity remains.

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    $\begingroup$ Most likely the other user deleted the comment. And this also was reason why your flag is marked helpful. $\endgroup$ – Martin Sleziak Dec 30 '17 at 16:05
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    $\begingroup$ Some words make a comment eligible for deletion after just a single flag. More information on Meta Stack Exchange. $\endgroup$ – Glorfindel Dec 31 '17 at 13:34
  • $\begingroup$ @Glorfindel: Thank you, but this was not the situation with that comment. $\endgroup$ – Alex M. Dec 31 '17 at 15:43
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    $\begingroup$ It could be that they (accidentally) used a word which is on Stack Exchange's blacklist. Anyway, a moderator will be able to see if the comment was self-deleted or deleted by flagging. $\endgroup$ – Glorfindel Dec 31 '17 at 15:46
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    $\begingroup$ I had kind of the reverse situation - I was the poster, told a commenter why the comment was not relevant and proposed to remove their comment. Next time I check, both comments disappeared. $\endgroup$ – domotorp Dec 31 '17 at 21:32
  • $\begingroup$ After Todd Trimble's answer the @Glorfindel's explanation seems quite plausible. (Or at least the information displayed to mods seems to be same as in such cases.) $\endgroup$ – Martin Sleziak Jan 5 '18 at 14:46
  • $\begingroup$ @MartinSleziak: It seems that the algorithm is roughly the following: user A posts something, user B writes a comment under this post, user A answers with another comment containing a ping @B (which allows the SE software to interpret this last comment as an answer). Now, if user B deletes his comment, user Community automatically deletes user B's answer. What happens if there are several comments by the same user? I don't know, and the SE staff is not very transparent about its software, unfortunately. $\endgroup$ – Alex M. Jan 5 '18 at 15:08
  • $\begingroup$ @AlexM. Maybe your last comment has some typos but what you describe there seems to be different from what you describe in the question. Did you perhaps meant to say "user A's answer" rather than "user B's answer"? (If I follow you correctly, in this example user B = Alex M. and user A=Chad Groft.) $\endgroup$ – Martin Sleziak Jan 5 '18 at 16:03
  • $\begingroup$ @MartinSleziak: Yes, that sentence should be "Now, if user B deletes his comment, user Community automatically deletes user A's answer". $\endgroup$ – Alex M. Jan 5 '18 at 16:16
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    $\begingroup$ @AlexM. I have posted a question about this on Mathematics Meta: Are comments which respond to another comment deleted by single flag? Feel free to ping me here or in chat if I have misrepresented something or if you prefer to ask the question there yourself. $\endgroup$ – Martin Sleziak Jan 5 '18 at 17:08
  • $\begingroup$ @MartinSleziak: Your question faithfully describes my situation, so let's wait and see if anybody answers it. Please comment here if this happens, thank you. $\endgroup$ – Alex M. Jan 5 '18 at 17:14
  • $\begingroup$ Since the explanation suggested there so far are the ones which does not seem to fit here (comment deleted by the author or by a moderator), my best guess is still that it was deleted because of some blacklisted phrase (as originally suggested by Glorfindel). $\endgroup$ – Martin Sleziak Jan 13 '18 at 6:16
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    $\begingroup$ @AlexM. Now there is an explanation posted on Meta Stack Exchange: meta.stackexchange.com/questions/305410/… It seems that it is better to avoid misspelling of Lipschitz. :-) $\endgroup$ – Martin Sleziak Jul 26 '18 at 17:40

The related question on Meta.SE popped up today and I looked into this to see if I could figure it out. Looks like the misspelling of "Lipschitz" lead to a word that triggers single-flag deletion appearing in the comment, which was why the no longer needed flag immediately deleted this comment.

For completeness, my answer on Meta.SE:

In this case, the mystery is clarified when you look at the content of the comment.

Deleted comment reading "Yes they are. Remember the maps go from larger n to smaller, and the map Xn+1→Xn has Lipshitz constant 1/2. – Chad Groft Dec 29 '17 at 22:57"

While it's not obvious at first, the comment was actually single-flag deleted because it contained one of the words that triggers immediate deletion - "shit" - inside the name "Lipshitz".

I've tested this to confirm the behavior.

So, there's not really anything new here, just an overenthusiastic search for keywords.

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    $\begingroup$ So any comment containing an offensive substring will be deleted automatically? $\endgroup$ – Asaf Karagila Jul 27 '18 at 12:43
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    $\begingroup$ Only if it's flagged, but yes, that seems to be the case. It's worth noting that there's no penalty for the comment poster when this occurs. $\endgroup$ – Catija Jul 27 '18 at 13:18
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    $\begingroup$ Is there like a list of these banned words somewhere? $\endgroup$ – Asaf Karagila Jul 27 '18 at 13:18
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    $\begingroup$ We don't publish them, no. There's a user-compiled list here, which is the closest thing available. $\endgroup$ – Catija Jul 27 '18 at 13:21
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    $\begingroup$ Thanks! I guess I will open a separate account, reach to 50 points, and then just post a comment with every single word in the English language, including common misspelling, variants, and slang, then use my main account to flag them and see what gets deleted automatically... or I just go back to my research. $\endgroup$ – Asaf Karagila Jul 27 '18 at 13:23
  • $\begingroup$ Could you perform an edit to restore the comment, or did you do this already? $\endgroup$ – Todd Trimble Jul 30 '18 at 2:09
  • $\begingroup$ @ToddTrimble I can not. Only comments that are deleted by diamond moderators may be undeleted. Anything deleted by the comment OP or by a single or group of users flagging can not be undeleted. $\endgroup$ – Catija Jul 30 '18 at 2:20

I don't have a full understanding of the specific process which was at work here, but the action was performed by the Community User which runs automatically in the background. When it saw your flag that your respondent's comment was no longer needed, it did the work, I guess when it saw you deleted your own comment right before.

  • $\begingroup$ So this would probably still be consistent with the explanation suggested by Glorfindel. At least, based on what I read here, it seems that single-flag deletions are handled by Community User: Can't undelete a comment which was automatically deleted by Community. (And on the comment it should say that it was deleted by Alex M. $\endgroup$ – Martin Sleziak Jan 4 '18 at 16:11
  • $\begingroup$ Here is the quote of the relevant part of the linked post: On the post's timeline, it shows that the flag was handled by Community, and that the comment was deleted by [name of the user who cast the "too chatty" flag]. When I view the deleted comment, it says "deleted by [name of the user who cast the "too chatty" flag] 12 hours ago". $\endgroup$ – Martin Sleziak Jan 4 '18 at 16:11
  • $\begingroup$ Martin: yes, this is correct. Although in this case, it wasn't a "too chatty", it was "no longer needed". $\endgroup$ – Todd Trimble Jan 4 '18 at 16:22
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    $\begingroup$ I have asked on the main meta: Are comments which respond to another comment deleted by single flag? We will see whether somebody knows about an alternative explanation. $\endgroup$ – Martin Sleziak Jan 6 '18 at 10:30

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