This is a very important question to ask in customer relations. Some customers (usually a few out of the lot) will be unreasonable, unruly, dishonest, over-demanding and show total disregard for your business terms and processes.
When an incident occurs in your business and you address it with the default mindset of “the customer is always right”, you are likely to end up with a team of disenchanted and unhappy employees.
Particularly, when it has to do with an issue between an employee and a customer, ask “what happened?”. The principle of fairness should not be skewed to the disadvantage of the employees. The more employees know that you will not hear them out, the more customer service issues you will have.
Seek to resolve the concerns of the customer, but do not be unfair and unjust to your employees in your judgment.
Asking “what happened?”, communicates to your employees that you also care about their feelings, dignity, protection, and not just making money.