Parental Controls in Apple Mac OS X 10.5 through 10.5.1 contacts www.apple.com "when a website is unblocked," which allows remote attackers to determine when a system is running Parental Controls.
http://www.vupen.com/english/advisories/2008/0495/references
http://www.us-cert.gov/cas/techalerts/TA08-043B.html
http://www.securitytracker.com/id?1019363
http://www.securityfocus.com/bid/27736
http://secunia.com/advisories/28891
http://lists.apple.com/archives/security-announce/2008/Feb/msg00002.html