My only question would be, does it normally come with mayo and ask for no mayo and substitute honey mustard? I can see how they'd screw that up on a regular basis. I suggest you admit defeat on getting the sandwich you want and stop for a moment (I take my food to a table or to the condiment bar) to check your sandwich before you leave. That 30 seconds of inconvenience is well worth it when compared to an inedible lunch.

And I think more people need to learn the art of the apology. An authentic apology can undo so much damage.