In restaurants in the United States, the custom is that anything brought to the table that you didn't order is complimentary. For example, after you place your order, the waiter returns to your table with a basket of bread. The bread is provided at no extra charge. These complimentary items are usually small, like some bread or a one-bite appetizer. (If anything bigger is brought to the table that you didn't request, it is customary to ask the waiter, "Is this ours?" just to make sure it wasn't delivered to the wrong table by mistake.)
In Portugal, the custom is that these items (known as couvert) are brought to your table as a convenience so you don't have to order them, but you still have to pay for them if you eat them. Again, they are typically small items like bread or a small plate of olives. It took me a while to adjust to the Portuguese custom, and I would absently start eating them before realizing that they weren't complimentary. Fortunately, the mistake is not costly; these couvert items usually cost only about one Euro, two tops, and they are things you usually wouldn't have minded ordering anyway.
There was one restaurant in the touristy restaurant part of Lisbon which was clearly trying to exploit the inattentive foreigners. Before my main course was ready, the waiter brought a not insubstantial cheese plate to the table. The cheese appeared quite old. (Yes, I know cheese is supposed to be old, but this one had dried out at the edges.) I may even have detected a layer of dust on it. This was the last meal of my visit, so by then I had figured out this couvert thing and knew to stay away. Which was a good move, because I checked the menu on the way out, and the cheese plate costs a whopping €7.50.
Another restaurant custom different between the United States and Portugal is the doggie bag, In the United States, it is customary for restaurants to offer to pack your uneaten food for you to take home. This is common because portion sizes in the United States have grown to ridiculous levels. The term doggie bag appears to have fallen out of favor, though. Now it's just called a box, as in "Would you like a box for that?" or "Would you like me to box this up?" The doggie bag is not part of Portuguese custom; when asked for a box to take home uneaten food, the waiter reacted as if this were a completely unheared-of situation. To his credit, he did find a solution: He went to the Chinese restaurant next door and took one of their take-out containers.
Nice to know that Chinese restaurants are the same all over the world.
Bonus restaurant tip: In my experience, in Portugal, you have to ask for the check. In the United States, whether you have to ask for the check or whether it will be brought to you varies regionally. In the Northeast, you have to ask for it; in the Seattle area, it will typically be brought to you, usually with a remark from the waiter like "Take your time," which means "I'm not trying to rush you out of here; I'm just saving you the trouble of asking for the check. Pay when you're ready." (Unless you're in a Vietnamese restaurant, in which case you have to ask for the check, per custom.)