An example from the English Wikipedia were common usage is overruled by offensiveness: 'Eskimo' is a very common name for a people who live in the North American Arctic. In fact it is probably the most common name for them (at least in the U.S.). But they consider it to be highly offensive. Therefore our article about them is at [[Inuit]] (a widely used alternate name - esp in Canada).
Unfortunately for this example, not all Eskimo are Inuit, and by and large anthropologists still stick with the more accurate definition, regardless of social stigma. (Thankfully, Nelson Mandella hasn't been re-styled an "African-American" on en yet. I'm waiting for the day...)
-- mav