That looks like a regression. I'm pretty sure that footnotes used to display properly in single brackets in posts. The editor preview is sometimes different (something about the client-side markdown library versus the final rendering, I think). Does anybody else remember footnotes having single brackets sometime in the past?

I can't remember noticing footnotes either way in the past, so I don't know. The post where I noticed it was one I posted recently on Scientific Speculation. I'd be surprised if a regression/bug could affect only one site but mentioning just in case. I'll also link to the specific post as I realise I'm making the assumption that it affects all posts - I haven't looked for other examples.

I think it's a safe bet that if it affects any post it affects all of them; this is rendering code, nothing community-specific about it. I just don't remember them looking like this in the past, and now I can't remember when (or if) that was the case.

I've just been looking through some posts on Writing for an example, and the only one I found was a fake footnote using <sup> for superscript. No examples of a markdown footnote. I'm sure I've seen one at some point but I can't think which community it was on.

I can't say with certainty that it was only single brackets when I saw it in the past, but the double brackets surprised me, which suggests it might be the first time I'd seen it.

In case it's useful in investigating this, I've happened upon a post with a footnote that correctly has just single square brackets.

I notice that it was posted 2 years ago, and has never been edited, in case either of those things are relevant. My guess would be that the age of the post is more relevant than it being unedited, as the example I gave in the question had the problem before any edits.

I realised that when I posted this Meta question I neglected to link to the example where I first saw the double brackets, so I have now edited to included that answer and the one I found today as examples.

The third example I linked to from the question was edited by Moshi shortly after, and now shows the double square brackets whereas it did not previously.

Suggests this may be related to the date and time that the post was saved, and may be stored incorrectly in the database rather than being a rendering bug.

Still shows correctly with single square brackets in the edit preview though, so it's probably not quite as simple as I'm suggesting.

Is the resulting HTML also stored in the database, in addition to the raw markdown? I'm struggling to think how else the rendered posts would only be incorrect for some save dates, and not others. If that's the case, then I guess that narrows the bug down to the code that produces the HTML to save in the database at the point a post is saved. I haven't looked at the code though, so these are just my rambling thoughts.

trichoplax‭ I think you're on to something. The database schema says that the posts table has columns for both "body" and "body_markdown". I'm guessing that we save the HTML to speed up rendering, but I'm not sure. If so, it would make sense that the HTML gets re-saved on any edit, and that would be why we see the time-based inconsistencies.