User blog comment:Rappy 4187/Technical Update: August 31, 2015/@comment-1398253-20150903010950/@comment-5558012-20150904231514

"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."
 * Not always. I've seen them within the page on the source tab before. It's not consistent but it happens sometimes.