I was like wtf at first so I calmly asked for an example. He said one customer has come back disappointed with their drink however, you are currently at the register about to serve someone. I said I would communicate with the customer in line I apologise however I will be back in a short moment then I would accomodate for the other customer's dissatisfaction, apologise and thank them for letting us know.
It's best to pretend like you don't have enough cash to change the note or you can't open the till. Don't accept the note but make up an excuse so you don't embarrass the customer. Ask if they can pay by EFTPOS instead.
If there is a customer taking long time at the queue to decide what she/he wants to order and there are another 20 customers waiting behind, what would you do in that situation and what would you to tell her/him?