That One Word
A few years ago I visited an exhibition of John Singer Sargent paintings with a classmate. In the final gallery was this portrait of Sibyl Sassoon (later Marchioness of Cholmondeley).

I had known since childhood that the name Cholmondeley is actually pronounced "Chumley". People love to share this piece of trivia at the slightest opportunity. I was not exactly innocent of this habit, and as we stopped at the portrait, I told my classmate, "did you know that name is pronounced 'Chumley'"? A moment later, a stranger passing by proceeded to share the exact same piece of trivia with us.
How many times a day did this painting inspire the exact same interaction? How many people, after walking through the exhibition, learned how to pronounce 'Cholmondeley', and left with an ardent desire to share their newfound knowledge with the world?
There are other words like this: off the top of my head, "Liverpudlian" and "Glaswegian". According to this graph, Liverpool has the highest rate of insurance fraud in England. Oh, those Liverpudlians! You know that's what they're called, right? Liverpudlians. And the interlocutor finds himself overcome by the desire for a strong, anaesthetizing drink.
It starts to become annoying, especially once you begin to notice the pattern. What other words are like this? Are there any that aren't British place-names? Or, if you know a language other than English, are there equivalents in that language?