From CF:
"I used a hopper on a few blocks like my Smoker, Blast Furnace, and Furnace that goes into a double chest. Because of the hopper I didn't get any XP from the furnaces etc as I never took items out, the hopper did. However, every time I picked the base up the XP would drop but it wouldn't lose the xp. Every time I pick it up it duplicated the current XP in the furnace and dropped it out, place base down, pick up, xp. Essentially it causes an XP duplication for infinite XP and it only gets more and more each pick up the more you smelt without taking stuff out yourself."