If you place items into blast furnaces using hoppers and then go into spectator mode to look into the GUI, the items in the blast furnace will jump into the first 3 slots of your inventory (smelting item in slot 0, fuel in slot 1, output in slot 2) and push the rest of your items across by 3 (slot 5→slot 8, hotbar.8→poof).

How to reproduce:

1) Place an item into a blast furnace using a hopper
2) Go into spectator mode
3) Look into the blast furnace
4) You will see the items move into your inventory about 75% of the time