I was just answering this question. It is about a use of "should". The word seems to have undergone a semantic shift away from a simple first-person form of "would". Instead it is today most often limited to expressing an obligation or expectation, to a degree that the old use is now prone to misunderstandings.
Are there other changes of this speed and degree or is such a quick change rather uncommon?
Edit: "should/would" was only an example. The question is aiming at changes in the language which happened so fast and are so profound that fairly recent, mid-20th century texts can be misunderstood.