10. The customer is often wrong
The first rule of retail: good customer service doesn't always mean ensuring the customer gets his or her way. No, just because one item was hanging on the sale rack does not mean it's on sale. No, if an item has no price tag it is not free (and please, as a customer, save yourself some dignity and never make that joke). What you learn here is how to choose your battles and how to use common sense. Someone trying to return a shirt covered in deodorant stains with the price tag in the pocket and claims it's "never been worn?" That's cause for alarm. If someone just wants to make an exchange or get store credit for something they lost the receipt to, whatever.