Using really different languages (Like Latin for writing books and speaking the local European language) wasn't unusual in the past, and it isn't that unusual now, e.g., for scientists in non-anglophone countries writing in English and talking in their national languages.
But the situation for Old Church Slavonic vs. Slavonic vernaculars can be described in different terms: There is a formal register for writing, and there are other registers for speaking. This general situation is pretty universal, but the amount of difference between the formal written register and typical vernacular registers can vary a lot, between having the written register as an almost foreign language (High German in Switzerland comes to my mind as an example) and having very little difference between written and spoken language (typical for languages that have a rather short written history). When the difference between written and spoken language becomes to large, there are sometimes quite radical shifts and reforms of the written standard (Modern Greek with its shift from Katharevousa to Dimotiki comes to my mind).
Just for the sake of completeness, I want to mention alloglottography: In Ancient Persia, public inscriptions were written in a completely different language, but read aloud in Persian by the scribes.