@Steven **************
at Mae Sai you can be denied "exit". They won't let you out on a day pass to Tachileck if they're not going to let you back in. For example, if you have done two land crossings and try a third, as far as I heard, they won't give you the day visa into Myanmar because when you have to come back they wont accept you. They eill instead tell you to go somewhere else asap.
That is my understanding anyway.