Wikimedia Bugzilla is closed!

If you have a template that's on a bunch of pages. Then you add a category link
to that template that is dependent on an argument. The category will not be
added to the pages that include the template.

Example:

[[Template:Foo]] (before):
 Some template

[[Bar]]:
 blah, blah, blah
 {{foo|42}}

[[Template:Foo]] (after):
 Some template
 [[Category:Useless {{{1}}}]]

In this example, after editing [[Template:Foo]], [[Bar]] would not be added to
[[Category:Useless 42]] until it was editted.