In one week of flying across the country, I must've seen atleast five folks being turned back at the gates on account of their carry-ons not being tagged or the tags not being stamped by security. I hope none of them missed their flights.

And you know the really funny thing - the harried security personel at the gates entrusted with this critical duty do not even seem to check the actual date on the stamp. In that one split second, the only thing they see is whether there IS a stamp. If I were to save a stamped tag from a previous flight and display it at the gate, no one's going to notice (not that it would have any implication on the fact that my carryon gets x-rayed each time!).