The Oxford Style Manual (which C. gave me for my birthday!) has the best and most interesting translation list between American and UK English which I have yet encountered. From it, I've learned all sorts of things of which I'd had no idea. Here are some of the most unexpected.
Chicory and endive mean the exact opposite of each other in American and UK English. Chicory is endive, endive is chicory, but they are still different plants.
A mezzanine is not a floor up from the ground floor in a UK theater - it's underneath the stage.
In the UK, a trapezoid has no two sides parallel; in the US (of course!) it has one pair of parallel sides.
One I knew but hadn't consciously thought about before: a semi is a kind of house in the UK and a kind of truck in the US.