In 1.6.4 (Mariculture-1.6.4-1.2.1e.jar), if you place unknown magnesiish dust in the furnace in the first slot and titanium ingots in the second, the furnace processes the first dust, outputs a salt, then it continues to burn fuel over and over without processing any more of the dust. If you remove the ingots, it works again.

This isn't a bug. When it processes it, if there is something already in the tank, and it tries to make it, if the output is not the same fluid it will fail. It' s a way to force the same fluid output from items that give a random fluid.