In reading a question over on Physics.SE (I can't quite remember what it was about, something to do with rocket stages and launching over water) I came across a link to this picture:-
Over on the west coast there is a shaded area dubbed 'ET Disposal Problem'.
At first guess I would say it's an area in which a jettisoned external tank would have a higher chance, than "usual", of hitting a populated area/land in general.
Am I correct? If so is there anything else notable about it?