Some years back, we flew in via San Francisco, filled-in the two-part card and handed them the "Entry" part.
The idea being that, when you left, you should hand them the "Exit" part.
After a few days in SF, we flew domestic to LA to get the flight to Auckland.
At LA airport we didn't go through any barrier between domestic and international and boarded the Auckland flight.
We merely showed our boarding cards (issued at SF) at the boarding gate.
When I got home several flights later, I was still in possession of the the "Exit" card.
The US might therefore think that I am still illegally in their country.
I do hope it's all a bit more secure by now.
Nonetheless, unless they've repealed the Patriot Act, they can chuck visitors in jail for months without trial if they imagine that they might be terrorists or a bit shifty.
That could be a bit inconvenient so, being that I haven't officially left the US, I don't want to confuse them by going in again, so I shan't be risking it.
Ever.
