Categories don't react well to their members moving between special-cased namespaces (Category ↔ File ↔ (other)).
This was probably never noticed before Ic93616a5 because people rarely use categories that contain both files and pages. However, categories very often contain pages and sub-categories. (Also, no one in their right mind would move between File and other namespaces, while accidentally creating a category description page in a wrong namespace is not impossible.)
- On pl.wp, a user mistakenly created [[pl:Odmiany herbu Brodzic]], categorizing it under [[pl:Category:Herby szlacheckie]] (with sortkey 'Brodzic'), then moved the page to [[pl:Category:Odmiany herbu Brodzic]].
- When you look at [[pl:Category:Herby szlacheckie]], the subcategory is not shown and the page claims there are 72 subcategories.
- Click "next 200" at the bottom. Our subcategory appears *first* on the list, and the heading order is messed up ('B', ' ', 'A', 'C', …). The page claims there are 73 subcategories now.
- Click "next 200" at the bottom again. Our subcategory disappears and all is well again.
It is worth noting that the last member on the first page of the category is 'Borch III', which sorts before 'Brodzic' (the sort key for the problematic subcategory).
I'm not sure exactly what happens, but it looks like the status of the category member as regular page vs subcategory is not consistent (it sorts like a page, but displays like subcategory?).