Let me say here that this is not an odd thing to do. In fact, three of the four people in line in front of me had also left to get toast/fruit/etc. It only takes two or three minutes while the omelettes are cooking.
I come back for my omelette. When I get it, it looks a little odd; then I realize that he gave me someone else's with similar contents (mushroom, cheese, and tomato) which could easily be mistaken for mine. I go back to let him know that I have the wrong omelette.
His response is to scowl at me and say, rather rudely, "Well, if you'd stay in line you'd get the right thing."
I'm sorry, but I've worked in customer service, and the correct response, even if you think the customer is in the wrong, is a polite, "I'm sorry, let me make you a new one." I am always polite when ordering food, and I don't appreciate being treated rudely in return.