What I expected to happen was...:
I was expecting to cook some cobble to make a bunch of repeaters. (Also, the furnace was attached to a comparator since I have an item detection thing installed)

What actually happened was...:
When I put the coal and cobble in, the furnace gui shut down. The furnace cooking animation was running, but when I opened it again, both coal and cobble were missing (The comparator was off). Not only that, despite the fire animation being on the little fire thing that goes down over time was empty.

Steps to Reproduce:
1. Place furnace and attach comparator to the back
2. Place any 2 items into the furnace (fuel and something to cook)
3. The Gui will shut down and the items will be gone