@Jay ************
you would NOT get an overstay fine or stamp if you waited to go back to the immigration office a day or so after the date on an under consideration stamp.
NOW if you went to LEAVE the country after the date on the under consideration stamp you WOULD be on overstay and would pay the fine, although if you fly out usually an overstay of under 24 hours are not charged a fined you just get a thai langauge stamp next to your departure stamp.
The O/P is going to go back to the immigration office to get their loop closed on the U/C stamp not exiting the country