If you found this error while running Drupal behind a proxy or HTTP accelerator.

Only variables should be passed by reference

You will need to check this part of your settings.php file.

 'reverse_proxy' => TRUE,
 'reverse_proxy_addresses' => array('127.0.0.1'),

Be sure to have 127.0.0.1 with IP of where your HTTP accelerator or reverse proxy like (Squid or Varnish) is installed, otherwise you will see errors, and boost will not save the page.