Do you have Launchpad repositories into your list and your package manager is complaining about missing keys?
It looks like (finally) Launchpad repositories have the possibility to sign their packages and all you should do is adding the specific PPA key to the apt’s trusted keys list.
You can simply copy the key from the error message and get it from the Ubuntu’s keyserver.
Here’s my one-liner to solve the problem:
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com MISSING_KEY
