The obsolete 'site_offline_message' variable is not always deleted during the D6->D7 upgrade

Because the update function that deletes it looks like this:

function system_update_7036() {
  // Get existing roles that can 'administer site configuration'.
  $rids = db_query("SELECT rid FROM {role_permission} WHERE permission = :perm", array(':perm' => 'administer site configuration'))->fetchCol();
  // None found.
  if (empty($rids)) {
    return;
  }

// ... do other stuff ...

  // Remove obsolete variable 'site_offline_message'. See
  // update_fix_d7_requirements().
  variable_del('site_offline_message');
}

So if you hit the early return, the code that deletes the stale variable never gets run.

Should be a simple fix - just move that code out of this update function and into a new one. I'm holding off on writing a patch for now, though, since this is likely to conflict with #984358: Upgrade from 6.x to 7.x always pulls your site out of maintenance mode immediately which is already well on its way to being committed.