CiviCRM sometimes unsets $_SESSION during hook_user

I suspect that CiviCRM is clearing the session variable in some situations in hook_user() $op = "update". Therefore the following code in hook_update_N (http://api.drupal.org/api/function/hook_update_N/5) will sometimes die:

$_SESSION['my_key'] = 123;
$account = user_load(array('uid' =>567));
user_save($account, array('my_key' => 890));
if (!is_set($_SESSION['my_key'])) {
die('You've got a big problem!');
}

The problem is intermitent. The problem went away when I disabled CiviCRM. Meaning that the issue is either CiviCRM, or some other module in my installation that plays with the $_SESSION var in hook_civicrm_pre() or hook_civicrm_post(), but I believe that I ruled out the later possibility.

This could be related to CRM-3015