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?