In 2009 at Mini Siam they mistook my Vietnamese (at that time) fiancee*) for Thai, so I paid a bit less than I expected. I didn't make a fuss about it, having learned enough about face in Asia (at home I would have corrected it, done so often enough), but about 15 minutes later a "lady" came after us to check whether my fiancee was indeed Thai and she lost her temper (big time) when I readily admitted she wasn't Thai and refused to pay extra. She completely lost it when I told her it was her problem and to get the police if she thought she had any chance whatsoever after which she decided it was a lost cause and left us in peace. And no, the police didn't show up while we were there (another couple of hours).
*)currently my wife and mother of our two sons