I am American born and raised, and one of the most boggling differences I find with American vs. British English is when talking about clothing.
What Brits call trousers are what we refer to as pants, but I believe pants are what they call women's underwear.
What we call suspenders are what Brits call braces,. Suspenders to a Brit are what hold your stockings up, also known as garters in the U.S. Correct me if I am wrong, but garters to a Brit are what hold socks up.
A sweater in the U.S. is what Brits call a jumper. A jumper in the U.S. is a type of dress worn by women
Another different I am always cautious to point out to people is the usage of the word "pull".
In the U.S. if someone does something another person is known for doing, one might say "They pulled a <insert person's name>" For example, my coworker Sandy is known for being overly dramatic. If I were to display such uncharacteristic behavior at the office, one might say I "pulled a Sandy". However, to a Brit, to "pull" is to pick someone up, like in a bar, for example. So it's easy to see how someone saying I was "pulling a Sandy" might give the entirely wrong impression.