Step to reproduce:

1. Build a kiln multiblock
2. Place a hopper pointing to it's side
3. Place a redstone block to lock the hopper
4. Place 65 fuel items (blaze rods, etc.) into the hopper
5. Destroy the redstone block and open the GUI of kiln
6. The number of fuel items increases to 64, then drop back to 1
7. Open the hopper and there's nothing