I once when younger offended a white American, by using the term "Negro" and "Fag". Now bear in mind, I'm Australian, when I said "fag" I mean a cigarette and when I said "Negro" I wasn't aware of how offensive it was in an American context.
In Australia and the UK where slavery was abolished much earlier than the USA and where there was never formal segregation (ignoring mistreatment of indigenous populations for the moment) the term negro doesn't carry much offensive and really it's rarely used. It's like the blackface skit on channel nine, we as a culture find that less offensive because we don't feel guilty for slavery/segregation, so we make fun of everyone equally, that said the skit wasn't really funny to begin with.
Also an interesting thing is Negro is offensive in Italian, but simply black in Spanish.