there's a load of furnace issues i found with the latest snapshot so i'll list them all below
-nothing will smelt
-if you exit the furnace window with an item in the top slot of the furnace it and/or something in the bottom slot it will return to your inventory screen as the same number it was before or it will come back with a red 0 next to it
-if you shift-click anything in the top/bottom slot it will return as 0
-if a block returns as 0 from the furnace it will disapear from your hotbar even in creative

that's all the bugs i've found with furnaces so far, i will submit more bugs if i find any