Worst I've done is around 20 miles past empty. Never again!
When I did fill up the car it acted oddly. Because the tank was so empty I think it pulled some gunk up into the engine. I got all kinds off errors on the dash. I thought I'd goosed the engine. It wouldn't start properly. I was real worried. However, after a few more attempted starts the engine did eventually fire.
Moral of this tale. Never run on vapours.