1. The customer is never right (I've worked in retail since I was 14, and have never seen a customer complain and be right.)
2. The customer will never read directions, then mess something up, and blame you.
3. Don't bother putting small print on a sign, no one will read it. If they see a sign that says two for one, and then beneath that in smaller print; Tropicana Orange Juice, the orange juice part will be ignored, and the customers will think everything in the store, and maybe the store next door is two for one.
4. SERIOUSLY, THE CUSTOMER IS NEVER RIGHT. Most people are stupid these days.
5. Do not make eye contact with a customer when you're closed down, counting your drawer. They take that an an invitation.