When using the furnace to smelt items, some items don't produce the correct result. For instance, iron ore makes a bowl, porkchops make cooked salmon, and clay balls make an egg. However, some (at least one) recipes are correct. Sand still makes glass. I haven't tested anything else. All that is required after changing packs is to close out of MC and reopen the game. This fixes the glitch until the next pack change.