I wrote a little while ago about things that are correct that most people think are wrong. (Like correct apostrophe usage, sometimes.) Well, I thought of two more. And since this can be a regular feature, it got its own label.
First, I think I've written before that the preferred English-language way of referring to the nation most people call "the Czech Republic" is "Czechia." (Preferred by the Czech government, that is.) I have trained my kids correctly, but I know anyone else who hears us is going to think we're nuts.
Second, a (singular) schedule for running a meeting is an agendum. (And all the other Latin plurals, like stadia and data.)