You called a Scotsman an Englishmen. That's almost as bad as a white dude calling an African-American the N word.
Not really, its akin to calling somebody from your part of the world a Yankee, amplified by a couple of orders of magnitude as much war and hostility. It matters most at a football game (FA/SFA ensure no GBR team) , but we do like to make a big song and dance about it.
Never ask a foreigner where he is from. If he is not English he will tell you within a few minutes, and if he is English, why embarrass him?
The camera adds a thousand barrels. - Steven Colbert