process_bug.cgi in Bugzilla before 2.14 does not set the "groupset" bit when a bug is moved between product groups, which will cause the bug to have the old group's restrictions, which might not be as stringent.
http://www.redhat.com/support/errata/RHSA-2001-107.html
http://www.iss.net/security_center/static/10478.php