Comment 57 for bug 541595

Had issue with 11.50.1074 to 12.01.1532 upgrade:
Opera update errored out, Libqtgui4 failed
12.01 update manager would not work or ubuntu software center
Error indicator said try apt-get...played with that and went through the steps similar to post #46- Richard Jones (alison-richard) wrote on 2012-06-07:
Solved using the following terminal commands:
> sudo apt-get clean all
> sudo apt-get update
> sudo apt-get -f install
> sudo apt-get autoremove
After update completed install (500 packages) install is fine.

Opera issue was a bad public key...