In survival I have an automated furnace. 
Chest on top, hopper, furnace, hopper, chest. The on the back of the furnace I have a hopper with a chest on top of that. 
It works great, except... I always put 3 buckets of lava in the back chest/hopper for fuel. 
First on goes in, drops to the chest below, rinse repeat..

But for some reason once its all done burning, I am only left with 2 buckets in the bottom chest and the last 1 is gone? Not on the floor, not in any of the chests, not stuck in a hopper.. just gone.. 

It seems strange that it would work and pull the other buckets down etc but the last one just disappears. I thought maybe the first time was a glitch or I just only used two buckets etc but I tried again and for some reason the last bucket, once it pulls the lava from it and looks to drop down into the hopper below just isn't there... You can watch it pull from the furnace and then nothing. The previous 2 buckets are in the chest below, but the 3rd one is just gone...