As an American I am usually embarrassed by other Americans when traveling. In the UK you can always find the American on the tube or in the restaurant because you are bound to hear their conversation over anyone else. I was once enjoying afternoon tea in my local shop in Cambridge when a tourist family sat down and the man (thinking he was a worldly person I suppose) started (loudly) laughing at these American girls who he had stumbled across in Italy putting ketchup on their pasta. Well he had already marked himself as one who could not assimilate well just by forcing his conversation on the rest of us, but to top it off, he then loudly complained that there were no English Muffins on the tea menu... (For those of you who have not done British tea an English Muffin is essentially a Crumpet, kind of like you do not order 'French Fries' in France you do not order an 'English Muffin' in England).
But then I went on holiday in places that were lacking in Americans and had the pleasure of seeing how horrific Europeans can be abroad. On a trip to Rhodes my father and I accidentally found ourselves booked to an all inclusive resort. The place was full of rather rotund brit & germans who were all bald and sun burned. They got up, had about four trips to the breakfast buffet and then drank and lounged by the pool all day. I am not sure a singe one of them bothered to go to the Old Town to see the sights. Also when I was commuting between London and The Hague in the Netherlands I was subjected to many bachelor parties on their way to Amsterdam for the weekend. I can say with complete authority that anything an American does that is annoying abroad a Brit does as well. My friend James (who is British) did not believe me until he was a Brit traveling in America and got embarrassed himself!
This CNN blog pretty much confirms that everyone stinks at being a tourist: http://www.cnngo.com/explorations/life/tell-me-about-it/who-are-worlds-worst-tourists-291643?hpt=hp_c2
My annoying tourist story is definitely the not very bright man in the tea shop- what is yours?