"Ground floor" vs. "first floor"
It is ground floor in GB and first floor in the US.
The first three levels top to bottom are:
US:
Third Floor
Second Floor
First Floor
GB:
Second Floor
First Floor
Ground Floor
These are usually interchangeable, but I was recently visiting a relative at a major hospital on the shore of Lake Erie and when I came down the elevator I pressed "G" and found myself one floor below the one with the main entrance/exit. So, it's not entirely standard.