Oddly, entries in the category table are only created when a category gets its first member. The creation of a Category: page does not create a row in the category table, but it should. This causes http://www.mediawiki.org/wiki/Talk:API#Generator_.2B_categoryinfo_bug
Version: 1.14.x
Severity: enhancement
This bug is still present in 1.18. Witness the two queries:
PD-VAR is a category with an existing category page, but without any category members. The API does not return any categoryinfo.
Same query with PD_Finland, a category with members: the API does return categoryinfo.
This is causing a subtle bug in HotCat, which uses categoryinfo to figure out whether a category is a HIDDENCAT. Both categories are hidden, but since only one returns a categoryinfo, HotCat sees only one of them as hidden.
The workaround for HotCat would be a bit convoluted: get the contents of MediaWiki:Hidden-category-category, then check whether the category specified by this system message is in the categories of the checked category. Unfortunately, getting MediaWiki:Hidden-category-category would be one remote request more...