The Online Certificate Status Protocol (OCSP) service in the Security Framework in Apple Mac OS X 10.4 through 10.4.8 retrieve certificate revocation lists (CRL) when an HTTP proxy is in use, which could cause the system to accept certificates that have been revoked.
http://www.vupen.com/english/advisories/2006/4750
http://www.us-cert.gov/cas/techalerts/TA06-333A.html
http://www.securityfocus.com/bid/21335
http://www.kb.cert.org/vuls/id/811384
http://securitytracker.com/id?1017298
http://secunia.com/advisories/23155
http://lists.apple.com/archives/security-announce/2006/Nov/msg00001.html