Because of the way that wpautop works, I would expect that if you had a newline, or or maybe a space in your URL. I can’t seem to get a space or newline to save from the admin interface, but it is trivial to insert by editing the WP_HOME and WP_SITEURL constants into wp-config.php… but that causes every page to 404 or worse.
There must be something happening when the link is generated– possibly something hooked to excerpt_more or the_content_more_link. It is also possible that your theme is parsing the more link itself and bypassing the normal channels.
Without more information, that is all I have.