Is the customer always right, when are they wrong?

When I used to work at Dollar General my manager said a customer called and said that I was being rude.. because they were deaf which wasnt to my knowledge and I chuckled, pulled out my wallet and smiled and showed them my ID because I needed it for their alcohol as they appeared under 40, weird but she said "Dont worry about it, its not like you are supposed to know that they were deaf" so.. its a good laugh once they leave the store 😂