It happened to me several times that a comment I made was no longer there after a question was closed as an exact duplicate. It is hard for me to tell what exactly is going on because I can't see or reconstruct the comments anymore, but it seems that the mere mentioning of the word "duplicate" in a comment causes the engine to remove the entire comment as soon as the question is closed for duplication. I don't think this is the desired behavior, since any added content beyond the auto-generated comment could be useful for later visitors or the OP even after the question is closed.

I don't know if the moderators can see deleted comments with their tools, but if so, here's an example, here's another one and here's one on meta where it happened.

So my questions are:

What exactly is going on and can this be corrected? If this is the desired behavior, what's the rationale behind it?

We can't see deleted comments, but I've seen this happen, and I think your guess is correct. – Qiaochu Yuan May 25 '11 at 9:34
Thanks Qiaochu, this answered one implicit question of mine. – t.b. May 25 '11 at 9:38
up vote 5 down vote accepted

Any comments containing a single URL to the closed-as-duplicate-of question are automatically removed at the time of close.

Assumption being, these comments will be of the general form …

I think this post is a duplicate of

… and thus not particularly useful once the close occurs; at that point the duplicate question has been edited into the post body itself as:

Closed as duplicate of

Rendering such comments moot. If you have some other commentary to add about the close target question that you believe should survive the close, I recommend entering your comment thusly:

In case anyone wonders, 2) is addressing my comment to question 12345, which is a duplicate of the present question.

Thank you for the clarifications, this tells me how to avoid automatic deletion. Just to make sure I got this right: The first example I mentioned was of the form "The question and the accepted answer of the duplicate don't directly address the current question, but @xyz's post there answers it.", so it was deleted. Had I written "The question and the accepted answer of the duplicate don't directly address the current question, but @xyz's post there answers it." instead, the comment would have remained? – t.b. May 25 '11 at 9:59
@theo any comment with a single url containing /12345 where the interior number is a match to the ID of the closed question will be removed at the time of close. – Jeff Atwood May 25 '11 at 10:23

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