I work in retail and provide good customer service and always help customers.
Last week I had one go in to a full blown rage as we did not have any hand sanitizer and it was apparently all my fault. So I said dont speak to me in that manner and turned around and walked away from her.
Therr are many other moments I could mention but we would be here all day 
So what is the rudest thing a customer has done or said to you?