1. Fettuccine Alfredo. Americans will sometimes almost turn violent (something not difficult for many of them) when you tell them that pasta Alfredo and Alfredo sauce is not Italian. It is also overwhelmingly bad for you. I have already held forth in my usual learned manner here and here on this important topic. Not that this will stop them from persisting in the error of their ways.
2. Chicken parm etcetera. The real deal is eggplant parm (melanzane alla parmigiana
or parmigiana di melanzane). Chicken parm, veal parm and so on are a take on this, and are due to Americans' great love of meat. Some Italians even believe that the original did not come from parmigiana but from the dialect word parmiciana (slat). So that the original eggplant dish didn't even feature parmigiano. It does seem weird that a Southern Italian veggie like eggplant would be paired with a Northern Italian cheese like parmigiano. Even though I am an eggplant, I don't know if this is true.
3. Spaghetti and meatballs. Spaghetti is Italian and meatballs are or can be Italian, but not together. Again, this is due to the American penchant for meat. Don't tell anyone, but I prefer Swedish meatballs.
4. Italian subs. Italians do not have enormous sandwiches stuffed with meatballs, cold cuts and so on. Italian sandwiches are smaller and may not even have meat, and they usually don't mix all sorts of cold cuts together. They also feature fresh crusty bread.
5. Pepperoni pizza.If you say this in Italy, you will get a pizza with bell peppers (peperoni) on it. A sort of pepperoni pizza exists in Italy, called either pizza alla diavola or pizza al salamino piccante. It is not one of the more popular varieties of pizza.
(in the photo, Bertolli's Alfredo sauce. Although Bertolli is Italian, this is only for foreign markets).