Debian testing hardly breaks anything unless one does a blind dist-upgrade instead of upgrade. The normal "apt-get upgrade" attempts to install the latest software versions unless the process requires the removal of other software (except the older version of course) so that (newer versions bugs aside) it won't break the system. The "apt-get dist-upgrade" will attempt to install the latest version even though it could force the removal of other packages, which can completely bork the system in many ways (missing libraries, etc). When I use it I often do a normal upgrade followed by a dist-upgrade and check all packages it would uninstall before allowing it to proceed, if I do. I've burned myself multiple times before learning this:)

Ironically, the "dist-upgrade" must be explicitly called on the command line, but is the default setting in Synaptic (called "smart upgrade" there). Synaptic is a GUI package manager most newbies would probably use in place of the command line, so I would expect its default settings to be more on the safe side.