A thing I've noticed is that people seem to get more abusive and less tolerant whenever they're in groups. In CoD4, with the party system, often you get two teams chiefly populated by large parties of three or more.
Usually one side is American, and the other British (Though there are those who speak German, French, Italian etc, but I don't clash with them very often), and throughout the game there's a constant fight between them, and it starts to get as tense as the bloody World Cup.
Anyway, my point is, if I wasn't in a party, or they weren't in a party, we'd probably always get along. Whenever I meet an American on their own, when I'm on my own, neither of us feel threatened and we have a good game, but whenever I'm in a party with my mates, and we say anything, even if it's something like, "Well played lads," straight away we're showered with abuse about being British.
I must admit my mates do it to them, too, but personally I like to think that if I'm nice to someone, if they're worth bothering with, they'll realise and not shout at you for no reason. ^_^
My favourite insult I've been called is a 'Silly tw*t c**tmonkey cheating whore'. It was by a 10-year old, so drew almost incessant laughter from me for several minutes...