There was nothing wrong with any of the image files, yet the article was in CAT:MISSFILE. The problem was caused by some odd <nowiki>...</nowiki> tags, none of them properly opened and closed; removing them was the solution.
A Cite error: There are <ref group=a> tags on this page, but the references will not show without a {{reflist|group=a}} template (see the help page). message was present, and I spent some time looking for unclosed ref tags, but could not see anything was wrong. The solution was to spot check the article history and find where the error first occured. It turned out to be the addition of {{:List of Supernatural episodes}} in Special:Diff/841123985/next that caused it, I've never seen that before, and when removed in Special:Diff/849970482 things were back to normal and no cite error was reported.
