Ubuntu package management is kind of fixed for particular versions. Whenever I add a repository I should indicate codename of my system. But what if the package was released a while ago and older system release is expected? Or vice versa, the newer version of a package is available only for newer releases.
So, what if I add to sources.list repositories indicating "wrong" codename? For example, I have Xenial, but add ppa repositories for Yakkety and Oneiric? What will happen then?