Microsoft Internet Explorer 8, and possibly other versions, detects http content in https web pages only when the top-level frame uses https, which allows man-in-the-middle attackers to execute arbitrary web script, in an https site's context, by modifying an http page to include an https iframe that references a script file on an http site, related to "HTTP-Intended-but-HTTPS-Loadable (HPIHSL) pages."

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-2064)