I used WordPress 3.6.1 in debug mode, WP-Statistics 3.2 all running on PHP 5.4.19.
When I browse, I strip my HTTP headers of the
HTTP_REFERER header, and if WordPress is in debug mode, I get the following notice:
Notice: Undefined index: HTTP_REFERER in wp-content/plugins/wp-statistics/includes/class/statistics.class.php on line 125
I took a quick look at the code line in question and it seems that it assumes that
$_SERVER['HTTP_REFERER'] will be set.