Mac, private server, minecraft snapshot 13w09c
place a furnace, and a comparator as if to test for the contents of the furnace,
as in the picture, then put an acceptable fuel and thing to cook in the furnace.
when both slots are filled with appropriate items, you are forced out of the inventory, the items are lost, and the furnace becomes a burning furnace, even though it is not cooking anything.
Basically replicate the first picture, then put a log in the top and bottom of the furnace, and you can replicate the bug
MC-9018 Faceless Furnace/Furnace Duplication.