This is a hidden category, i.e. it is not displayed at the top of member pages among normal categories.
Users can choose to see hidden categories in a separate "Hidden categories" list by checking "Show hidden categories" in Special:Preferences#Appearance.
This is a maintenance category. It contains pages which pass an invalid value to the DISPLAYTITLE magic word. The value of DISPLAYTITLE must be equivalent to the default title: only capitalization changes to certain parts of the title (not all).