A comment on an Italian sausage thread brought to mind the confusion that can result, especially in international travel, when two different things get called by the same name in different places. That one was about pepperoni, which is a spicy dry sausage here but a green pepper in many other countries.
Others that come to mind:
A martini. Order this in a bar in France or Belgium and you'll get a glass of vermouth - if you want what English-speakers think of as a martini, you need to order a martini cocktail.
Andouillette sausage. In the US it's a garlicky pork sausage, in France it's made from chopped pig's colon and is very much an, um, acquired taste (as I found out to my dismay in Paris a few years ago).
And of course there are the common ones like potato chips (flat & crispy in the US, french fries in the UK).
Even staying here at home we have things like the sloppy joe, which in most of the country is a loose mix of ground beef and tomato sauce served on a hamburger bun, but in New Jersey is a cold deli sandwich made with cole slaw.
Any other more obscure good ones to watch out for?