I've had to challenge a bar tab or two. Some of the go-go bars are run by real slimesters. Always be careful to count your change. In fact, verbally acknowledge the demonimation of the bill you are giving them ("Here is 1,000 Baht") so that they don't come back and claim you only gave them a 500 note.

By the way, what - or where - is Boy Toys in Chiangmai. I've never heard of that place.