Never display the interviewer that you handled the customer badly, tell them that you tried to help them but still the customer is not satisfied. Example: Once I got a chance to face the couple who were in an argument and they are in a bad mood. I tried to help them, I gently asked them what they would like to have. They were very impatient at me, but still I maintained my pleasant smile and took the menu as per their instructions, when people are at bad mood they will not like whatever we do so all one need to do is just understand their mood and carry out the same professional behavior.