At first sight, the waiter’s remark could appear a little confusing. The idea of irony and sarcasm is what makes this situation humorous. The man expected the server to apologize and take the fly out of his soup when he first complained about finding one in it. But the waiter’s answer is meant to be funny since it turns the complaint into a different kind of conversation.
The waiter’s snarky remark suggests that if the man keeps complaining about the fly in his soup, it might end up becoming a common request among the other customers. Put another way, the waiter is jokingly speculating that the man’s grievance is so out of the ordinary that it would spark a crazy trend in which people start asking for flies in their soup—which is obviously not something that people would genuinely want.
Essentially, the waiter’s response is a clever attempt to minimize the gravity of the issue and use humor to ease the tension that the complaint had created.