(warning: this post may contain failed humour)

The problem -- and I think there is now wide consensus that it is a problem -- is that deleted comments are not currently visible to anyone!

Happily, this is no longer completely true! We still can't see deleted comments through the web interface, but I can now download a dump of the database, which does contain all deleted comments. It's a minor hassle to make and process a full dump to do it, but it means that I can relatively easily reconstruct full comment threads. w00t!

I wanted to post a new answer. I could have simply edited my first answer, deleting all the previous content and replacing it with the new. However, I judged that unethical since the previous question already had 12 votes. Unethical not because of reputation, but because the 12 votes were for the previous answer, not for the new answer. So I posted a completely new answer, making it community wiki to avoid any issues of reputation.

The question remained as to what to do with the previous answer. As it stood, it wasn't answering the question and was beginning to attract votes against because of this; plus, as I've said, the change in tone in the question made the answer seem to have a negative tone. All of the relevant information in the answer had actually been taken up into the question so there was no point in leaving it lying around. So I deleted it. I said what I had done in the new answer.

I would not object to the answer being visible so long as the original question was visible at the same time. Namely, if someone is looking at the original answer then they are also looking at the original question. What I would not like is for someone to read that question and not be able to see instantly what the original question was.

Ideally, one should be able to step a whole question-and-answers back through time, rather than one piece at a go. Questions-and-answers on MO are part of a dialogue, and it's not fair if part of the opening statement gets changed but the initial response not. Deletion may be a sledgehammer, but sometimes the flimsy nutcrackers just don't work.

If anybody has posted a meaningful response, then you should probably leave it up and post an addendum, like "please ignore my previous comment" if it's a comment or "Edit: nevermind, this is wrong" if it's a proper post. The fact that somebody posted a meaningful response probably means that the only harm in leaving it up is to your pride (which you probably overvalue). It also means that somebody put in the time to think about whatever you said; deleting your comment/post after they've done you that favor is likely to annoy them quite a lot. Of course, you'll have to decide what "meaningful" means; if they've just pointed out that your whole argument is rubbish in a boring way, they probably won't mind too much if you delete. The point is that it's rude to waste somebody else's time, and you should keep that in mind when deciding whether to delete something.

If you really want to delete something you posted, I think you generally should be able to, but I think Andrew has overstated his case a bit (perhaps intentionally to mix things up a bit). "I want the right to signal that I no longer stand by what I wrote and, at the moment, deletion is the clearest way to do that." Why not just post another comment or edit the post to explain what's wrong? There may be a good reason, but I think there usually isn't. I don't know what the situation was with that answer with 12 votes, but it sounds like enough people thought there was something valuable that it might have been worth it to keep the answer up. A line at the top saying "Edit: this is wrong because XYZ" does the job of signaling that you no longer stand by what you wrote, it makes it unlikely that anybody would waste their time by thinking that the answer is correct, and has the benefit that it's not going to annoy anybody who thought there was something of value and didn't want the post deleted.

If it is felt that it is unacceptable to delete stuff, then it should be possible to "give it away" instead. That is, I should be able to remove my name completely from the post (so making it "community wiki" doesn't qualify).

The issue, for me, is more about ownership than about content. I'm quite happy for my deleted posts to be visible but I wish it to be clear that I no longer judge these posts as fit for this circumstance. Indeed, perhaps I should put it more strongly than that: I delete a post if I consider it to be actively harmful to someone browsing the question (for example, if it may distract them from a better answer). I have no problem with posts being left up "for posterity" so that "others may learn from my mistakes" - I have no mathematical shame! - but I would like it clear that these are scraps from the notepad and should be treated as such, no matter what their ranking in the views (I deleted one answer that had 12 votes and had clear water between it and the next answer).

If there was a way to designate answers as "correct" or "incorrect" and ensure that incorrect answers never rose above correct answers, then I would be happy to use that instead of deletion (except in rare cases where deletion is absolutely the right thing to do).

I want to have control over what I have written on MO. I wrote it, after all, and it is my name that it links to. Even if I wrote it in good faith, later circumstances may change its interpretation and I may decide that I no longer wish it to be placed in that position. At this point, though, we enter the murky waters of who owns what on MO. I have copyright on all that I write there, but I agree that others can take it and share it and "remix" it, subject to suitable attribution. Where this gets complicated is identifying the original source of the material and identifying "remix"es. In particular, if I post something and someone else edits it later, are they remixing my post? Is "my" post the original source in the history and the edit belongs to the editor? Is the fact that my name still appears viewed as "attribution" or is it a bit disingenuous, since it does look - to the casual observer - as though it is still my post.

As a daft example, suppose I post:

As I've come to realise, Anton is completely correct here.

and someone comes along and edits it, say changes "correct" to "insane", who does Anton sue? Me, or the editor? And, more importantly, who does it look to the casual observer that Anton should sue?

If people are worried about continuity, perhaps deleted comments should be replaced with the text "A comment was deleted". But since what I write is inexorably linked to me, I want the right to signal that I no longer stand by what I wrote and, at the moment, deletion is the clearest way to do that.

Despite the fact that I did delete that answer, I can't help agreeing with everything David says. And I wouldn't really have minded leaving it up; I just would have edited it so that it began with the sentence "Warning: this answer is wrong".

I suppose there's a possibility that if Anton and co. rule that wrong answers shouldn't generally be deleted, then some people will be too shy to post.

Somebody pointed out that I overindulged, and in some cases it might have been true. I think it is bad etiquette to delete your trail. It makes others who followed up on your comments look like jackasses. I realize this, and so I think it might be a good thing to put up somewhere(probably an etiquette page, linked from FAQ).

But I do think that the privilege to delete comments should be there. We might want to retract something and this is the best way.

And, if somebody remembers any answer of mine whose deletion is felt to be inappropriate, I am willing to undelete it. After doing a brief checking, I still feel that I would rather keep them deleted, since there is some problem with each.

As far as I remember, my feeling at the time was that my answer wasn't wrong in any kind of useful way. I didn't think that anyone would learn from my mistake. So, it seemed pointless (and slightly embarrassing to me) to leave it there. The situation was about as simple as could be: mathematical mistake; no emotions visible.

I am not sure if we have to worry too much about this issue. The events David mentioned seem to be isolated, and most MO users, to me, have been acting in good faith. To be fair, I have fortunately not been involved in any heated discussion, so may be my view is biased here.

@CSiegel: I did that in the recent case that Prof. Speyer is alluding to, yet that did not help the situation.

Suppose I make a post on MO which I regret. Either (1) it contains an error or (2) it offends another user. When should I delete it and when should I apologize and correct it?

I almost always do the latter, in both cases. (Examples of (1) and (2).) I find it very frustrating to come back to a conversation and find that comments which I remember have been deleted. I realize that MO isn't meant for conversations but I think that, when there is already a risk of confusion or offense, it is best to return to the conversation based interaction which humans have used for millenia.

I believe that some members of the MO community disagree with me.

I think that, perhaps after we have discussed this some, it would be very useful for Anton to make an explicit ruling. Etiquette works best when everyone knows the rules, and miscommunications in this area have caused conflict already.