Opera 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."
http://www.securityfocus.com/bid/35403
http://research.microsoft.com/pubs/79323/pbp-final-with-update.pdf
http://research.microsoft.com/apps/pubs/default.aspx?id=79323