User blog comment:Rappy 4187/Technical Update: August 31, 2015/@comment-1398253-20150903010950/@comment-1398253-20150904230344

If it was accidental, I don't think the categories would have been removed, as, in the classic editor in the Wikia skin, they're on a side panel and not in the main editing window. So I don't think it was an accident. I don't want to accuse him of doing it on purpose if it is indeed a bug though. Something similar has happened before.