Mean. In the UK to be mean implies you are frugal to the point of being stingy. In the US you might be mean (i.e. aggressive) because of that English guy's inability to get his wallet out and buy you a beer (cv).
Autumn. My favourite time of year when the leaves turn orange, red and yellow. You call it 'Fall'. I prefer Autumn.
Candy. We call them sweets. Unless they are American confectionary, then we call them candy too. I have met quite a few Americans girls called 'Candy' but never ever an English one called 'Sweets'.
Cutlery. The impliments you eat with. You guys also call them flatware.
Sucker. In both countries a fool or a silly person. Also a piece of candy on the end of a stick that us Brits call a lollipop or a lolly. We also call money 'lolly' too to make things just that little bit more confusing...
Z. The twenty sixth letter of the alphabet. You call it 'Zee'; we call it 'Zed'. A whole generation in England has had to relearn the alphabet after hearing the 'Alphabet song' on Sesame Street. Sadder still, the song doesn't rhyme with the English 'Zed'. At least the 'Numbers song' works (1-2-3-4-5, 6-7-8-9-10, 11-12, do do-do do-do do-do do etc etc...)
Tire. When visiting the garage make sure you know the difference between a UK tire (band of metal placed around the rim of a wheel designed to strengthen it) and a US tire (pneumatic effort called a 'tyre' in the UK). If you make a mistake it could be a very long and bumpy ride home.
99. In the US purely the number before one hundred. In the UK a yummy variety of ice cream consisting of a scoop of vanilla soft-scoop ice cream in wafer cone with a chocolate flake stuck in it. The cone is specially designed to allow the melting ice cream to flow all over your hand before you get to eat it.
Centennial. Dull but apt. You call the period lasting a hundred years a centenary.