If Unstable was enabled in the sources.list file, the corresponding packages will almost never be installed because their default APT priority is 1. This is of course a specific case, designed to keep users from installing Unstable packages by mistake. The packages can only be installed by typing

aptitude install package_name/unstable

Users typing this command can only be aware of the risks that they take. It is still possible (though not recommended) to treat packages of Unstable like those of other distributions by giving them a priority of 500.

This is done with a specific entry in /etc/apt/preferences:

/etc/apt/preferences

Package: *
Pin: release a=unstable
Pin-Priority: 500