Moodle 1.8.x and 1.9.x before 1.9.8 can create new roles when restoring a course, which allows teachers to create new accounts even if they do not have the moodle/user:create capability.
http://www.vupen.com/english/advisories/2010/1107
http://tracker.moodle.org/browse/MDL-16658
http://lists.opensuse.org/opensuse-security-announce/2010-05/msg00001.html