This pejorative should be shouted and/or typed at any moment when both parties involved in a conversation type or say what is essentially the same concept (with allowances for differences in wording).
The declaration should especially come into play when this confluence occurs at several moments in the same conversation; the first instance might be unique, but three or more would justify loudly shouting "BATFUCK AND MUSTARD" from any available rooftops.
B: I no longer require cantaloupes in my repertoire of melons.
A and B (unison): BATFUCK AND MUSTARD!
A: I need a beer.