When is the saying "the customer is always right" wrong?

I work in customer service and my job would be GREAT if it wasn't for the customers. When i go shopping i am always nice and polite even if something is wrong i always try to be nice because i know how it feels to get yelled at for something you can not control and there are times when i DO tell a customer they are wrong. Just tonight i had to tell someone they bought something and thought something came with it and i was like no they are two separate items but she was very nice and apologized and said she thought it was one item but then me being the way i am and since she was being VERY she got the deal which was buying 2 things for 10 dollars then got the third item she thought came with it that was 5 dollars and had a 5 dollar coupon and i told her she could use the coupon and still got that item for free. The coupon was for 25 dollars or more also and she was only spending 15 dollars then got the 5 dollar item for free. I am not rude to customers though when they make mistakes like that i just explain it to them and i always seem to get nice customers who understand every once in a while i will get the customer i wanna smack up side the head and be like wow your dumb!!! LOL but when i go shopping i am always nice and polite and i do not abuse "the customer is always right" saying because 99% of the time the customer is WRONG!!! I hate when people think they know my job better then i do and it annoys me! When you are in uniform getting paid and my manager then you can start telling me what to do and i am wrong until then shut your mouth and let me do MY job, because i DO {believe it or not} know what i am suppose to be doing and know MY job and MY responsibilities.

Hi there! As a customer, I always want the best when it comes to the goods and services I pay for. Customer value is very important for me. I am an entrepreneur (part-time, full-time teacher) and I agree that to some extent, the customer is always right. There are times when, I see customers who take advantage of this. It is not easy dealing with customers who think that they are always right. The customer is NOT right when he insists on things that are either impossible or those that he alone believes is right. For example, if a bar closes at 2am and a customer wants to extend just because he wants to drink, I do not think it is proper to keep the bar open just for him. Besides, keeping the crew for another hour or so means that they are entitled to overtime pay. If there's just one customer, I do not think that it would be practical to stay open just for him. He can't claim that he is right in this aspect. Well, if he wants more drinks, there are other bars where he can drink 'till he drops. Just my thoughts...

I am sure there are many times when the customer is always right wrong but it is better for a business to give the benefit of the doubt to the customer. I am not pushy, I prefer to leave a store if I feel that I have been wrong by the staff, if it happens again I just don't shop there.