@EM ****
as far as I know there are no written rules regarding visa exempt entries. Though if you enter too often or stay too long on Visa Exempt stamps some immigration officers suspect you are trying to live long term in Thailand on Visa Exempt stamps (or Tourist Visas).
What really bothers me is that it's kinda like russian roulette. It's completely random as to when someone is being questioned or denied entry into Thailand. Some people seem to get away with years and years of back to back Visa Exempt entries and others are denied entry at the 2nd time of trying to enter in a calendar year. Playing russian roulette is not a pleasant start of a holiday for me.