The problem comes with using only numbers as URLs. Here is a forum thread in WP that discuss this issue. I’ll cite Otto:
WordPress 2.3 and up does not allow the post or page slugs to be all
numeric. This is because that URL scheme will conflict with multi-page
posts.There is no fix. Change them to something else.
Alternatively, a plugin exists to allow this, if you give up on
multi-page posting:
http://wordpress.org/extend/plugins/allow-numeric-stubs/More info here: http://trac.wordpress.org/ticket/5305