Bugzilla 2.14 before 2.14.2, and 2.16 before 2.16rc2, allows authenticated users with editing privileges to delete other users by directly calling the editusers.cgi script with the "del" option.
http://www.securityfocus.com/bid/4964
http://www.redhat.com/support/errata/RHSA-2002-109.html
http://www.iss.net/security_center/static/9303.php
http://bugzilla.mozilla.org/show_bug.cgi?id=141557
http://archives.neohapsis.com/archives/bugtraq/2002-06/0054.html