The customer is NOT always right. In fact, the customer is, quite frequently, very often wrong AND a fucking idiot. In fact, such a big fucking idiot that he/she does not know that he/she is a big fucking idiot. This is known as illusory superiority, or the more specific Dunning-Kruger effect.
Go ahead, scream louder at me; it only makes you an even bigger fucking idiot.
The dumber you are, the surer you are about yourself.
If you know so much about my job, then why the fuck did you call me at 3 in the morning for your stupid fucking medical problem and then refuse to listen to me?
Whoever came up with that stupid phrase about the customer always being right or something like that was a gutless, spineless worm.