An independent test in 2009 uploaded multiple versions of the same song to YouTube, and concluded that while the system was "surprisingly resilient" in finding copyright violations in the audio tracks of videos, it was not infallible. The use of Content ID to remove material automatically has led to controversy in some cases, as the videos have not been checked by a human for fair use. If a YouTube user disagrees with a decision by Content ID, it is possible to fill in a form disputing the decision. YouTube has cited the effectiveness of Content ID as one of the reasons why the site's rules were modified in December 2010 to allow some users to upload videos of unlimited length.