pacman corrupted my installation twice in a year. Obviously I did something wrong but who knows.
It could get a bit tricky if you update very rarely, like once a year, but not sure what else there is to do to break things. Please elaborate a bit more.
Archlinux repo holds only a single latest version. There are some community maintained repos that contain older versions (and those can be easily fetched through downgrade utility).
When updating Archlinux, upgrade everything and then reboot. This way it should not break.
If you want to install something manually in arch it is better to create a package first (if it is not already in aur). That way pacman can check for corruptions before installing anything