I'm a college student who works at Subway, so I totally loathe the fact that people assume I'm not very bright based on my employment.
My favorite is when people speak very quietly and when I ask, "Tomato?" to confirm what I heard, they yell "MAY-OOOOOO!"
Sorry, bitch, tomato and mayo sound very similar in a noisy store. At least I asked before putting it on your sandwich.
Be grateful I don't mess with customers' drinks when they're mean to me! Something that seems harmless could actually be dangerous (i.e., giving a customer sugar-free syrup instead of regular... they could be allergic to aspartame). Maybe you asshole crustys should keep that in mind when you insult the people who handle your food!
I usually just smile, tell them to have a nice day, and add something along the lines of "please fall off a cliff you stupid whore," in my head.