When trying to update a category description in WordPress 4.1, I got this cheerful message in green:

Category not updated

In my case, I had a category and tag with the same slug. Changing to a unique slug allowed the form to be submitted successfully, and the category updated.

WordPress stores categories and tags in databases the same way, likely due to the fact tags were an afterthought. WordPress will generally try to prevent such unique ID clashes from occurring, but they must still slip through the cracks.

Interesting though that WordPress doesn’t enforce slug uniqueness at the database level.