[Taniwha]

It causes all sorts of other problems too - because you don't pass thru immigration as you leave the US you don't get a stamp in your passport, if the system fucks up (it's the airlines who become responsible for processing your exit), the US still thinks you're there, many many people have been caught this way, suddenly found themselves in some detention centre for 'overstaying' because the INS's bookkeeping is out of whack last time they were there.

ALWAYS travel to the US carrying the boarding pass from your last departure