When attempting to smelt Acacia and Dark Oak logs in a furnace, they simply will not 'cook' into Charcoal.

To replicate:
Place charcoal, coal, or any burnable wood item into the bottom slot of a furnace, and then place a Acacia or Dark Oak logs into the top bit.

(Shift-clicking Acacia or Dark Oak while there is another block in the bottom slot will not place it in the top slot either - another bug?)