I remember overstaying on one accasion accidentally, after Leaving from Heathrow on the 1st November arriving on the 2nd and completed 30 days flight booked on the 2nd of the next month l went through immigration at Bangkok Airport and got told to go to a desk behind immigration and a senior immigration official said to me l had overstayed l said how? When l stayed Thailand exactly 30 days?
He replied your 30 day visa exempt begins when you board your flight from London to Heathrow, l apologised and said l didn't know my 30 days begins when l board flight to Thailand,
Anyway he wavered the 500baht fee and well you know now, on your way.