When all other slots in a furnace are full, a hopper will put items into the result slot. This can allow infinite xp, by putting in, for example, iron ingots, and then taking them out repetidely.
Steps to replicate:
1. put items (not ingots) into fuel and input slots in furnace
2. place a hopper feeding into the furnace and put items that give you xp (for example iron ingots) into the hopper
3. remove the items from the furnace, giving you xp, and then put them back in the hopper.