If your flight is shortly after midnight be careful on the timing. A friend of my had inadvertently overstayed and went and cleared it at the airport and then went and had a coffee and a sandwich as he had allowed plenty of time for his 2 am flight. When he went to pass through immigration he was told overstay, he said he had paid and showed the receipt. The IO pointed at his watch and said one more day, it was 00.20am.

My friend said he had just spent the last of his baht and had none left. The IO disappeared and came back with a foreigner that paid the 500 baht, I could only assume it was some immigration assistant that had access to the 'kitty'.

Usually, you not only pay the fine in the office, but they also stamp you out after you paid the fine and got your Overstay stamp stamped into your Passport.

You would not have to go back to the line and counter for normal procedures, so I doubt your story.

I overstayed often for some days and know the procedure.

If the flight is after midnight it's already anyway a day more and they normally would have done that adjustment and payment of a day more, already at the Overstay desk - or overlooked it.

If your flight is shortly after midnight be careful on the timing. A friend of my had inadvertently overstayed and went and cleared it at the airport and then went and had a coffee and a sandwich as he had allowed plenty of time for his 2 am flight. When he went to pass through immigration he was told overstay, he said he had paid and showed the receipt. The IO pointed at his watch and said one more day, it was 00.20am.

My friend said he had just spent the last of his baht and had none left. The IO disappeared and came back with a foreigner that paid the 500 baht, I could only assume it was some immigration assistant that had access to the 'kitty'.

Sorry, But I think your friend told you a tale.

Prior to moving to Thailand I traveled to Thailand for work very often. As things go we always had delays and I would end up with a day or two overstay here and there. I can tell you 100% that you take care of any overstay after you go through immigration, so impossible for him to pay for an overstay prior to going trough immigration. When you go through immgration and they identify that you are on overstay they will have you go to an office to meet with another officer and pay your fine. When paying the fine the immigration officer will also review your boarding pass and if the flight is after midnight they will add another day to your overstay time.