A friend of mine was stamped in till March 31, and her flight is leaving April 1. I advised her to go to Chaengwattana (Bkk immigration office) to get a 7 days extension. So she did, but at Chaengwattana she was told simply not to bother: no problem. She asked about 4 times to reconfirm, and was told the same thing every time.

Stll feeling a bit uneasy about it (so would I), she decided to take a taxi to the airport straight from the hotel on April 1.

When leaving by air, a one day overstay is not fined. (If longer than 1 day you will need to pay the first day also). It is noted in the passport, but without consequences for a new visa or entry to Thailand.

I have done it myself.

It may not be automatic at land borders, but I wasn't fined for one day overstays at Ranong or Poipet.

I have overstayed quite a few times. That was back several years ago when I was doing the border run every 30 days. I just forgot what day it was! Paid my fine and it was no big deal...once in Poipet for a 2 or 3 day overstay and once at the airport for about the same.