When used on an image description or any other page, HIDDENCAT categorizes the page in [[Category:{{MediaWiki:Hidden-category-category}}]]. This magicword should only work in categories.
Version: 1.12.x
Severity: minor
URL: http://commons.wikimedia.org/wiki/Category:Hidden_categories
An example for an image with HIDDENCAT can be found at http://commons.wikimedia.org/wiki/Image:Tweeter_seda.jpg, the other files in http://commons.wikimedia.org/wiki/Category:Hidden_categories are directly placed in it due to a bot error.
Fixed in r31393; now only sets the hidden-category state if the page is a category page.