I edited a category that had an undefined tag set, and got the expected error message. The URL was still the one for editing that category, but the refreshed page had a "New Category" header:
I then, from this page, edited the tag set, and instead of being the edit suggested in the URL, it attempted to create the new category per the page heading. Note that the URL has now changed to categories/new from categories/11/edit:
I edited a category that had an undefined tag set, and got the expected error message. The URL was still the one for editing that category, but the refreshed page had a "New Category" header:
I then, from this page, edited the tag set, and instead of being the edit suggested in the URL, it attempted to create the new category per the page heading. Note that the URL has now changed to
categories/newfromcategories/11/edit: