Certain links will be parsed twice, once for MAGIC_URL_LOCAL and once for MAGIC_URL_FULL, giving an invalid link that, to the user, looks and works just fine. But if you view the source of the post, you'll see this:

<!-- l --><a class="postlink-local" href="http://www.phpbb.com/community/"><!-- m --><a class="postlink" href="http://www.phpbb.com/community/">http://www.phpbb.com/community/</a><!-- m --></a><!-- l -->

To try it out, just make a new post on this site containing, where the contents of the post contains the URL http://www.phpbb.com/community/
Then view the page source.

It seems to happen when the domain of the url matches the domain of the site and is followed by a subdirectory that ends in a slash.