Why was a user able to post a question with just the 'untagged' tag?

Question

Today on Literature, a user was able to post a question with the untagged tag by itself. If you look at the first revision of this question, you can see it:

And they did this on purpose. From the comments section:

what exactly did you tag this question with when you asked it? I'm trying figure out how it ended up [untagged] and I can't figure it out, because you can't tag your own question that... Thanks! – Mithrandir♦ 22 mins ago

@Mithrandir untagged - exactly that. – TheAsh 15 mins ago

Huh. That's... odd; the system is supposed to prevent that from being possible. That's strange. – Mithrandir♦ 13 mins ago

It works for me, too. I was able to post a question with untagged myself on Literature – I posted one and deleted it a moment later, just as a test. But when I try to post a question on other SE sites with untagged, I get an error that looks like this:

So I don't think it's supposed to work.

It seems like something is different on Literature. Could it have something to do with Literature having a special tag wiki for untagged? A tag wiki excerpt was created back in September. Did that cause untagged to become a normal tag which people can add, or is something else going on here?

Answer 1

I added the blacklist to all sites back in 2015.

Literature didn't exist then, so it didn't get the blacklist. We should probably get in the habit of adding it during site creation, but for now I've just added it to all sites created in the interim.

Answer 2

The tag untagged appears automatically when all of a question's tags are removed. When editing a question, it's impossible to remove all the tags, so the only way a question can get down to 0 tags is if its sole tag is removed by the system. This happens because there is an automatic job that removes tags that are used on a single question and that are 6 months old. Single-question tags are common on this site since most questions have a tag for the author of a literary work and a tag for the work itself, and it's natural to have many works and authors with a single question on the site.

A request to remove this automatic job, which has zero benefit and actively hurts the site, was denied by Stack Exchange.

There is a way to preserve a single-use tag: you need to create a tag wiki. This needs to be done before the tag is up for removal: the tag killer job does its work silently, without warning or notifying anyone or leaving any log of its action. The only way to observe it is to watch untagged grow.

Once untagged is created, there's nothing special about it. People can add it to their question like any other existing tag. The tag may have been blacklisted at some point, but evidently it isn't today. There's another automatic job that removes most blacklist entries that haven't been triggered in a while, so even if the tag was blacklisted once, it isn't today, and even if Stack Exchange added it back, it would get deblacklisted again in the future unless the deblacklisting job was turned off.