This vulnerability affects Opera 10.50.
XSLT is normally subject to strict controls, preventing documents from separate Web sites from reading the contents of other sites. Certain XSLT constructs can cause Opera to retrieve the wrong contents for the resulting document. These contents will appear randomly from the cached versions of any Web page that has previously been visited, and could contain sensitive information. This information can then be read by the resulting document.
Opera Software has released Opera 10.51, where this issue has been fixed.
Thanks to crazypops for discovering this issue.