Safari in Mac OS X before 10.3.5, after sending form data using the POST method, may re-send the data to a GET method URL if that URL is redirected after the POST data and the user uses the forward or backward buttons, which may cause an information leak.
https://exchange.xforce.ibmcloud.com/vulnerabilities/16944
http://www.kb.cert.org/vuls/id/128414
http://lists.apple.com/mhonarc/security-announce/msg00056.html