Tag autocompletion fails for anonymous users? On a site, anonymous can create content. The content type has a term reference field, which works fine for logged in users.

However, for anonymous, the fields JS is "dead", no autocompletion happens, and I can't even see an http request going off.

Obviously I assumed is has to be a permissions issue as it only affects anonymous, but I can't seem to figure out what permission that would be, and if it was, then it seems like the http request would go off.
Figured it out myself while I was typing the question.

Since JS didn't work at all, I checked the console, found a CKEditor related error, fixed it, and things went back to normal.