It has nothing to do with you, the customer. In my experience, the times I've caught fake money were from people who likely had nothing to do with it, I feel bad because are legally not allowed to give it back. I would recommend that everyone be familiar with how to recognize fake money, that way you won't accept it and lose money.
http://www.secretservice.gov/money_detect.shtml
http://www.secretservice.gov/money_detect.shtml