I traded with a Farmer VIllager to get him to max level, but when I placed a Lectern in a nearby area to start trading with a Librarian, this farmer permanently swapped jobs into a Librarian.

I can't replicate this glitch intentionally, but my word it's lucky that guy wasn't a Mending librarian or something! Now I'm hesitant to expand my village in case this happens to a more important Villager.