Although isn’t always specific to bar girls, it tends to be something which sticks in people’s minds and unfortunately tends to be associated with the bar world.

Probably due to the alcohol consuming environment.

If your bill is less than 500 Baht, after paying with a 1,000 Baht note, the waitress will return with your change. This is sometimes when you look twice, see you have been given change for 500 Baht, and say to her “But I gave you a 1,000 Baht note”

The reply can be one of two options. Either she will realise you did give her 1,000 Baht and go back to the cashier to sort out the error.
Or, she might deny it emphatically, saying it was only a 500 Baht note you gave her.
If you end up going down this avenue, it’s obvious the matter will escalate, whether to your satisfaction is another point.

An aside from the bar scene:
I remember once buying a ticket for the Pattaya – Bangkok bus when the price was 90 Baht, the only change I had was a 500 Baht note. Waited for my 410 Baht change, the ticket clerk only offered 10 Baht. When I politely pointed out the error, there was no sudden recognition of her mistake from the lady, she just gave me the remaining 400 Baht without looking me in the eye, not a word of apology.

The way to avoid this pitfall? Make sure you always have plenty of small change when you go out, especially if you’re out on the town for the evening. The chance of you having consumed a lot of alcohol can encourage any not-so-honest staff to try on this scam.