If you use a hopper to pull the 'cooked' item from the furnace you don't get any XP (which is fine), but the XP will 'build up' in the furnace.
If you cook lots of items then take the hopper away and cook just a single mutton for example you will then get all the XP that hadn't been given to the player for the previous items. I went from level 8 to level 22 by taking one mutton out of a chest.
Video demonstrating the issue: https://youtu.be/sUKqAB7aB54?t=616
It can be possible that the world crashes or even becomes unplayable because of the amounts of experience_orb entities created when an item is taken out of the furnace.