I just did an upgrade on my 1.9 test site to build 2007101533.01 and Moodle is now throwing the above notice hundreds of times on every page. It's this bit of code...

4718
4719 //error_log(" Record: " . print_r($user,1));
4720
4721 //
4722 // Pagination controls
4723 // Note that we might end up removing a user
4724 // that ends up not having the rights,
4725 // therefore rolling back $c
4726 //
4727 if ($lastuserid != $user->id) {
4728

Granted, it could be a local issue on my test site, but I thought I'd better report it to be safe!!