So, what happens if your root partition is full and you reboot your machine ? If it is really full, and in particular there is no space to write anywhere, you might be stuck with a no space on device.
To avoid this problem, there exists a script /etc/init.d/mountoverflowtmp that runs a check to see if there is a minimum acceptable space on /tmp and if there is not, it mounts it overflow. It also checks for unneeded overflow tmpfs for /tmp and removes them if that is appropriate.(src).
But if you do not reboot again, you might get stuck with a mini /tmp directory of 1Mb . This is a neat trick, but you need to know about it to avoid unpleasant and unexpected surprises.