Opera before 10.63 does not ensure that the portion of a URL shown in the Address Bar contains the beginning of the URL, which allows remote attackers to spoof URLs by changing a window's size.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A12135
http://www.opera.com/support/kb/view/972/
http://www.opera.com/docs/changelogs/windows/1063/
http://www.opera.com/docs/changelogs/unix/1063/
http://www.opera.com/docs/changelogs/mac/1063/