Opera before 12.01 on Windows and UNIX, and before 11.66 and 12.x before 12.01 on Mac OS X, does not properly escape characters in DOM elements, which makes it easier for remote attackers to bypass cross-site scripting (XSS) protection mechanisms via a crafted HTML document.
http://www.opera.com/support/kb/view/1025/
http://www.opera.com/docs/changelogs/windows/1201/
http://www.opera.com/docs/changelogs/unix/1201/