Is it right that customer is always right? if not, where do you draw the line?

This saying refers to genuine cutomers who are disappointed by our product/service.They are always right evev if they are angry ,even if they are mad, even if they abuse you.They are always right,because,the reason they acted in this bad way is because we did not keep our promis of delivering their expections.Now these customers can be given the status of being always right.
But there are also those shrud customers who would like to manipulate the system & grab benefits that they don't deserve.These kind of people should politly be declined