This is a tricky question. Customer has to think they are always right, to have enough belief in themselves to actually have a say. After all, they are paying. In reality, for the most part they are often wrong. To be right, one has to spend 20 years in related industry. Of course, you don't say "wrong" to the customer on first encounter. And definitely not until you spent some years in this. Especially true for support, every product is different and has a learning curve. Less of a problem for software development.