It is rude to intentionally dirty up someone's food.
Steve is a cook at a restaurant, and a customer just complained at him.
Steve wants to get the customer back.
Steve asks the customer to leave the restaurant because he's being hostile.
Steve purposely drops the customer's food on the floor in the kitchen.
The customer leaves the restaurant, upset but doesn't want to be embarrassed so keeps it to himself.
The customer ends up finding the dirt in the food and leaves a bad review on Yelp.