When apt-get returns an error message like NO_PUBKEY F9A2F76A9D1A0061, the problem can be solved by adding a valid key:

$ gpg --keyserver keyserver.ubuntu.com --recv F9A2F76A9D1A0061
$ gpg --export --armor F9A2F76A9D1A0061 | sudo apt-key add - 

In case of a missing key from Opera:

wget -O - http://deb.opera.com/archive.key | sudo apt-key add -