Take TopTex for example: the tag [bug] doesn't exist there, nonetheless `bug` comes up among the suggested tags when `b` is entered into the input field for adding a tag. This is confusing; it made me believe that the tag [bug] exists there, but I'm not able to add it to my question for some reason.
`bug` only comes up *below* the suggestions for the tags that actually exist, but when there are no existing tags that match the given prefix, the only suggestions will be nonexistent tags, making it seem like the nonexistent tags exist on the sub-site.
Screenshot of the behavior described above ("when there are no existing tags that match the given prefix, the only suggestions will be nonexistent tags"), for TopTex (which doesn't have [bug]):
This is browser autocomplete kicking in which I neglected to switch off for the tag input fields - now fixed.