The British national tourism agency issued some guidelines on dealing with foreigners in preparation of the 2012 Summer Olympics. According to them, extra patience is needed when dealing with people from India, Mexicans do not want to discuss current affairs and Canadian tourists will often be annoyed at being mistakenly identified as American.
Personally, I’m not offended when people mistaken me as American. Often, I can’t tell the difference myself. But I suppose some Canadians get irritated because a) they feel Canada is very different from America and b) Canada is often overshadowed by its more powerful neighbour. The funny thing is when I correct people and tell them I’m from Canada, they usually apologize for the mistake. Either they know Canadians are easily offended or they think it’s very offensive to be identified as American.