I was fiddling on a wordpress-plugin today when I encountered a strange error, the moment I submitted a form WP gave me a sweet little 404 error.
To keep things short, this was not directly related to the form submit or the POST-method used in it. After digging around in WP it came to light that there is a bunch of reserved variable names for queries. So whenever you submit a form or call a WP-url which returns a 404 and you are pretty damn sure that the url is correct check the wp-includes/class-wp.php file for the arrays $public_query_vars and $private_query_vars, these variables are your enemy in that case (see below).
When you know what to search for there is also a page concerning this in the WordPress Codex.
/**
* Public query variables.
*
* Long list of public query variables.
*
* @since 2.0.0
* @access public
* @var array
*/
var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots', 'taxonomy', 'term', 'cpage', 'post_type');
/**
* Private query variables.
*
* Long list of private query variables.
*
* @since 2.0.0
* @var array
*/
var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and', 'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and', 'tag_id', 'post_mime_type', 'perm', 'comments_per_page', 'post__in', 'post__not_in');