1. Cappuccino after a meal: Cappuccino is for breakfast. That’s it. You don’t order a cappuccino after a lavish meal because milk is hard to digest. Go for a simple “caffè”  (espresso coffee) instead if you don’t want to out yourself as the foreigner.
2. Ketchup on pasta: Ketchup is rarely found in Italian kitchens. It is served with french fries. But never ever with pasta.
3. Put parmigiano cheese on seafood pasta: Either you eat seafood or you eat meat/cheese/vegetables. Parmigiano simply doesn’t belong on fish or clams.
4. Have “Pasta Bolognese”: If you ask an Italian for pasta bolognese he would raise his eyebrows wondeing what you are referring to. Italians have tagliatelle with ragu (which can be with or without tomatoes). But they simply have never heard of Pasta Bolognese. The same is almost true for “Alfredo sauce”.
5. Ananas on pizza: There is no such thing as “Pizza Hawaii” in Italy. You simply don’t put Ananas (or any other fruit) on top of a pizza.

We could go on. How about you ? Do you want to add some ?