What speed bumps (pun intended--see photo) can you encounter in your own language as you rush through research?

We all take it for granted that we speak our own language and perhaps one or two or a half or so more. We can get along in our own country and maybe a few others. That works.

But we may get complacent and forget that research demands that we remain flexible about language usage in the context of time and place, because it is indeed dynamic, often culture-oriented, and ever-changing.
Even for those working in English only--
What are the words you need to know to research the British of 1860? The Welsh or Australians of 1890? If you read the American census records, do you know what a cordwainer is?

And what about the Canadians of 2015?

A "sleeping policeman" is a speed bump.