Not just slang (I mean not just intentional use of slang). I used to talk to a man online who was very interested in politics. Because I am American and he was not (and I think English was like his fourth language), he routinely referred to President Bush as "your Bush". I'm female, so this made me very uncomfortable and I finally asked him "Do you know what that means in English slang?" He wasn't attempting to use slang, but boy did it sound bad to me.
There are all kinds of landmines when you try to speak another language. I have lots of humorous/embarrassing stories from growing up in a bilingual family.
