function cimenweb_redirect_to_category( $template )
{
    if ( ! is_404() )
        return $template;

    global $wp_rewrite, $wp_query;

    if ( '/%category%/%postname%/' !== $wp_rewrite->permalink_structure )
        return $template;

    if ( ! $post = get_page_by_path( $wp_query->query['category_name'], OBJECT, 'post' ) )
        return $template;

    $permalink = get_permalink( $post->ID );

    wp_redirect( $permalink, 301 );
    exit;
}

add_filter( '404_template', 'cimenweb_redirect_to_category' );