There are some reserved word for register taxonomy. If you are passing the term through the $_GET or $_POST array. Doing so can cause WordPress to respond with a 404 error without any other hint or explanation.

Following is the code snippet to check reserved terms.

//Forbidden attribute names
//http://codex.wordpress.org/Function_Reference/register_taxonomy#Reserved_Terms
$reserved_terms = array(
            'attachment', 'attachment_id', 'author', 'author_name', 'calendar', 'cat', 'category', 'category__and', 'category__in', 'category__not_in', 'category_name', 'comments_per_page', 'comments_popup', 'cpage', 'day', 'debug', 'error', 'exact', 'feed', 'hour', 'link_category', 'm', 'minute', 'monthnum', 'more', 'name', 'nav_menu', 'nopaging', 'offset', 'order', 'orderby', 'p', 'page', 'page_id', 'paged', 'pagename', 'pb', 'perm', 'post', 'post__in', 'post__not_in', 'post_format', 'post_mime_type', 'post_status', 'post_tag', 'post_type', 'posts', 'posts_per_archive_page', 'posts_per_page', 'preview', 'robots', 's', 'search', 'second', 'sentence', 'showposts', 'static', 'subpost', 'subpost_id', 'tag', 'tag__and', 'tag__in', 'tag__not_in', 'tag_id', 'tag_slug__and', 'tag_slug__in', 'taxonomy', 'tb', 'term', 'type', 'w', 'withcomments', 'withoutcomments', 'year',);

if ( in_array( $taxonomy_name, $reserved_terms )){
    $error = sprintf( __( 'Slug “%s” is not allowed because it is a reserved term. Change it, please.' ), $taxonomy_name );
}