The bug

You can put any type of log into a furnace, open the recipe book and click on charcoal. Then every log will be transferred into the other type of log except one.

To reproduce

1. Have one log of type A in your inventory (for example, oak) and a stack of logs of type B (for example, birch)
2. Put the one A log into the top slot of a furnace (doesn't need to have fuel)
3. Show the recipe book
4. Press shift while clicking onto charcoal in the recipe book
   → The top slot of the furnace will be filled with as many logs of type A as possible, even if you don't have any more logs of that type in your inventory