The one that always confuses and sounds wrong to me is the American use of the word "momentarily" it seems to mean "back IN a moment"there, whereas in British English it means "back FOR a moment".
The other that gets up my snotter is Certificate pronounced" serstifficate" and the word "borrow" used instead of lend, as in " Borrow me your pen" as mentioned above, Guaranteed to get a sarcastic response that one, as in " From whom???".