Customers are always right they said,to what extent is this saying true?Justify this in your own opinion.

In a situation whereby a customer slapped you as a sales representative while you're trying to attend to him /her,but unfortunately there is another customer rightly in front of you before you can be able to attend to this customer in question simply because he/she was anxious and ready to take the turn of another customer.While you're trying to calm him/her with plea to attend to him/her next,he/she intentionally slapped you.To what extent are their right in this regards?