Its a myth that there is a forever happy and satisfied customer. To transform this myth into a reality you have to keep on trying. Installing a mirror behind the counter helps to change the mood of the customers by showing them how messed up they look with that lightning bolt on their forehead. When they see their angry face in the mirror, they calm down to look pleasant, and your day gets saved.
This is the key reason why big mirrors are placed behind the counters in bars, .