Opera before 11.60 does not properly consider the number of . (dot) characters that conventionally exist in domain names of different top-level domains, which allows remote attackers to bypass the Same Origin Policy by leveraging access to a different domain name in the same top-level domain, as demonstrated by the .no or .uk domain.
http://www.opera.com/support/kb/view/1003/
http://www.opera.com/docs/changelogs/windows/1160/