Giving change for Bht500 instead of Bht1,000 is very common and happens to me about once a week in restaurants, supermarkets and the Skytrain ticket offices. I suggest using the Skytrain station to get small change(but make them lay it on the counter!) or specify an amount ending in 900 at an ATM. At supermarkets and BTS, the cashiers count out the small change first..ie everything above the rounded 100, then take the one hundred bills up to 500 and then often "forget" the final 500 bill. You/they are so engrossed in counting out the 57.50 baht or whatever, you/they forget the big stuff.