Splitting off from bug 28540.
If you have a page with [[category:Foo| ]] on it, its expected to sort under an empty header (or one containing a space to be more specific). In the uca-default collation, it sorts under U+6DE character ( ARABIC START OF RUB EL HIZB ). Probably something to do with the weirdness of sorting things under a space. Might have to do some sort of special casing for it, since it is kind of a special case.