Older versions of CI can't handle random query string parameters. This is how you clean it out.

codeigniterfix.php

<?php
  // Place in index.php, at the top

  function clean_hnp() {
    $reset_query = false;
    $keys = array(
      'action', 'hnp_post'
    );
    
    foreach ($keys as $key) {
      if ( isset($_GET[ $key ]) ) {
        $reset_query = true;
        unset($_GET[ $key ]);
      }
    }
    
    if ( $reset_query ) {
      $_SERVER['QUERY_STRING'] = http_build_query( $_GET );
    }
  }
  clean_hnp();
?>