User relationship module - error strict warning: Only variables should be passed by reference
IMHO, this is a bug in the User Relationships module, and should be reported to that module's issue queue.

If you look at the documentation, you see that the template  for drupal_render is:

drupal_render(&amp;$elements)


Passing a function call by reference not good coding practice (but it will not have adverse effects if you never use the return value passed by reference).

The following will do the same thing and not produce the warning:

$_shift1 = array_shift($elements);
$_shift2 = array_shift($elements);
$_shift3 = array_shift($elements);
$rows[$rtid] = array(
  drupal_render($_shift1),
  drupal_render($_shift2), 
  drupal_render($_shift3), 
);


The module author probably doesn't have PHP-warnings enabled, and never saw the warning.