This has happened to me more than once.   As you know, the customers take numbers and we go by the numbers.  For the most part, it works.  Every now and then, I call a number, the person responds.  What happens is another customer walks up and says - "I didn't take a number because no one was standing here so I'm next".  Also, a customer will tell me what they want, and as I'm slicing it for them, they walk away.   Sometimes when I finish they have not returned.   So, I move on to the next customer.  While I'm helping the next customer, they return, get mad at me and they are like "I wasn't finished yet. I need some other stuff."  They have been gone for 5 - 10 minutes.  I'm not gonna stand there, especially on  a Sunday, and wait for you to return while all the other customers are staring at me. 

On Sundays. we usually have one employee to take care of the deli kiosk orders because there are a lot of them.  Some customers are like "How come that person isn't helping? I've been standing here for 20 minutes."   We explain that they are taking care of the kiosk orders.  So the customer is like "Why do they get their stuff before me and I've been standing here?" Duh. that's the idea.  Use the kiosk, and you don't have to stand and wait.  Or get from the grab & go. 

Also, I wish some of these customers would get the point that the grab & go is sliced fresh every day.  "Oh I want freshly sliced from behind the counter, I don't want leftovers".  IT IS NOT LEFTOVERS!!