Current limits usually make this Just Work: clamping diodes sink enough current to drop the voltage from 12V to 5V (or whatever) and the thresholds in the reverse direction are defined so that a 12V circuit still sees 5V as high if there isn't a terrible amount of noise.

Of course, this all depends on sane current limiting behavior from the 12V side. I'm sure there's at least one line driver out there that does a good "gnome swapping terminals on a car battery really fast" impression.