Which words or grammar forms are likely to cause a collision between American and British English?

For all the Mickey-taking on both sides of the water I suppose British and American speakers understand one another 99% of the time.

Can anyone think of any areas of vocabulary or grammar where serious confusion could arise?

The only word of which I am aware, which in America is the near antonym of its British meaning is nervy. To an American, saying someone is nervy means they have nerves of steel such as a high-wire performer. In Britain it means they are of a nervous disposition almost to the point of breakdown.

This question has been inspired by a European OP who has queried which form of English it is desirable to learn to avoid his worlds colliding.

Edit Clearly one could look up one of the many books on sale that provide translations from American to British and vice-versa. It was not my intention in asking the question, that we should compile yet another of those. What I thought would be interesting would be to hear of some actual instances of where you could get into serious difficulty, perhaps an anecdote or two.

I remember reading somewhere that the opposing meanings of the term "to table" on either side of the pond caused confusion at meetings between the British and American heads of state during the second world war.

edit: found it, from Winston Churchill's book The Second World War, Volume III, The Grand Alliance:

The enjoyment of a common language was of course a supreme advantage in all British and American discussions. The delays and often partial misunderstandings which occur when interpreters are used were avoided. There were however differences of expression, which in the early days led to an amusing incident. The British Staff prepared a paper which they wished to raise as a matter of urgency, and informed their American colleagues that they wished to "table it." To the American Staff "tabling" a paper meant putting it away in a drawer and forgetting it. A long and even acrimonious argument ensued before both parties realized that they were agreed on the merits and wanted the same thing.

A few examples right off the bat:

• The word spunk has a secondary offensive meaning in British English

• Pants are undergarments in British English and outer wear in American English

• Floor numbering is different. Brits have a ground floor, just like Germans, the Dutch and other European countries. In America, the first floor is on the ground.

• The C-word is a lot less offensive to Brits, although still shouldn't be used lightly

• The word spaz is a derogatory name for a person having cerebral palsy, or, as an insult, a stupid person in British English. In American English it means to have a freakout and is more often used as a verb

• (this came from some googling) the word quite has some hidden dangers ("quite good" might be understood as worse than just "good")

Bottom line: the most differing parts seem to be slang and swearing

I might be mistaken, but my impression is that in England, to "knock (someone) up" means to go to their house and knock on the door. In America, it means to make someone pregnant.

Also "pissed" in England usually means drunk. In America, it's short for "pissed off", which means "very angry".