Curing a zombie villager and then turning it into a librarian allows an exploit for infinite emerald duplication. The villager can sell 3 bookshelves for 1 emerald, you can break those bookshelves for 9 books, trade those books for 9 emeralds, and so on and so forth. The only limitation is the villager's stock, and by keeping the villager locked next to their lectern it continually restocks.