The problem happens when it's on default controller or default method. Take this example:

Let's say that the calendar class is running on the Code Igniter default controller and default method, and is accessed with the URL ``http://example.com/``. The links will be ``http://example.com/year/month``, so if the URLs are rewritten to remove the index page, the CI will try to find a ``year`` controller with a ``month`` method, otherwise the server will search the folder called year for a file called month.

If the calendar class is running on the default method, with the URL ``http://example.com/index.php/controller``. The links will be ``http://example.com/index.php/controller/year/month``, so the CI will look for the ``year`` method.

If the calendar class is not running in the default method, the links will be ok: ``http://example.com/index.php/controller/method/year/links``

Understand? I found this issue when I'm testing my changes, I forgot to set the next_prev_url and I'm running the calendar on the Welcome controller and index method, and when I tried to see the next months he gave me a server 404 error because ``http://localhost/codeigniter/2014/02`` isn't a file.

But I don't know any app with the calendar running in the default class (I have some websites with calendar running on front pages but the next and prev links use AJAX calls to other controller instead of normal redirects), so I don't have a app to defend this feature.