When logged in as guest, going to Activities -> Quizzes page triggers this php error notice:
Notice: Undefined variable: showing in C:\moodle\www\moodle18dev\moodle\mod\quiz\index.php on line 100
Variable $showing not initialized.
This has been corrected in moodle 1.9, line 56: $showing = 'scores'; // default
Please backport the initializing of this variable to 1.8.