Does checking in before midnight at Suvarnabhumi Airport prevent overstay penalties if my visa expires the same day?

That's what you'd expect; the stamping out by immigrations SHOULD be the single qualifying moment to determine overstay. The good news is that it indeed is, for many. The bad news is however, that some keep reporting that their immigrations officer determines overstay based on the date printed on their flight ticket instead.

There is more good news though: if this then yields one day of overstay, the fine is usually waived. However, if you were already on overstay passing immigrations, this extra day does matter. So then you'd hope that they use the 'common sense method' and stamp you out for the date they stamp you out on.

You allowed 1 day overstay

You don't need to worry about 1 day overstay. You won't be charged 500 baht for a 1 day overstay in Suvarnabhumi Airport. And the overstay stamp is no big deal. Those who think an overstay stamp can be a problem are clueless. In the list of all potential reason to be denied a visa or entry in Thailand, overstay stamp is very far down the list.

Yes

Yes I did the exact same thing last trip, no overstay😊

Yes!

No overstay!

I overstayed 1 day no fine no entry in passport just got a day mixed up..

I left officially approx 12 hours after my visa expired and they didn’t flag me for an overstay payment.

@Heidi *******

agree with

@Brandon ***********

yes, no worries either way. If one day overstay, they will stamp you overstay 1 day and likely waive the 500 baht fine. If thru immigration before midnite, no overstay

You will be fine. No worries

Yes, you officially leave Thailand once you pass through passport control and are stamped out of the country.