Don't put ketchup on a hot dog in Chicago: Really, this should go for any hot dog, anywhere. Ketchup belongs on French fries. But especially don't ask for ketchup in the hot dog capital of the world. Some places will straight-up refuse you; others will simply mock your ignorance. This doesn't mean you need to get the full-on salad that is a traditional Chicago dog. Get just mustard and relish. Get chili. Wrap it in bacon. But no ketchup. Ever.
Don't order a steak well-done, anywhere: The magnitude of this miscue increases with the quality of the steak in question, but the bottom line is a cow gave its life to bring you that steak (albeit not knowingly; it's a cow). Don't besmirch its memory by cooking it beyond recognition into a brownish wad of overcooked sadness. Some people say a juicy medium-rare steak is "bloody" and "gross". You're eating a damn hunk of cow flesh. There SHOULD be blood involved.
Don't use a knife and fork on pizza: There's an exception here for deep-dish, but, generally speaking, silverware is to be avoided on your garden-variety, highly foldable pizza slice, unless you're three years old and Mommy is cutting it into small bites for you. And, even then, it's borderline. And what are you doing reading this site, three-year-old? WHERE ARE YOUR PARENTS?!
Don't do any of the things you do at a sushi place: As with steak, the magnitude of the transgression increases with the quality of the sushi establishment. Don't rub chopsticks together to get rid of "splinters" -- it implies you think they use subpar chopsticks, and do you honestly know anyone who has ever gotten a mouth splinter eating sushi? Don't dump the wasabi in the soy sauce and mix it together (particularly if the place grates its wasabi fresh). Don't dump a bunch of soy sauce on a maki roll that the chef has already accompanied with an array of sauces and accoutrements. It's a wonder you aren't forcibly removed from sushi restaurants on a regular basis.
Don't double-dip: Yes, George Costanza helped America learn this lesson the hard way, but that's the thing about Americans -- we never learn! Double-dipping continues to run rampant, and those of you who think taking a bite, turning the chip (or whatever) around to the end you didn't bite, and dipping that end is some kind of workaround, IT ISN'T. You might as well dip your finger in the bowl. One caveat: groups of close friends who have all open-mouth kissed each other can call off the double-dipping rule if the whole table agrees to it.
Never order Swiss on a Philly cheesesteak: Some Philly cheesesteak joints swear by Whiz. Some won't serve it, preferring American or Provolone. But, no matter what, don't walk into Philly thinking you're going to show them how it's done with some fancy-pants Swiss cheese request. Do you want to end up like John Kerry? Well, actually, John Kerry ended up getting to be Secretary of State, but still. Don't do it.
Use your right hand when eating Ethiopian: Never had Ethiopian food? Too bad; it's delicious and a great option if you're dealing with vegan friends (also a great option: making new friends). The first step is to avoid the "Hey I didn't know they had food in Ethiopia!" joke. They do, and you aren't funny. But the whole operation involves scooping up piles of tastiness with flatbread (called injera) that you tear into pieces with your hands. It's considered rude to use your left. Don't worry, lefties: I am one of you, and I have managed fine.
