Variant 1:
1. Place a hopper on a furnace
2. Put some coal blocks in the hopper
Result: The coal blocks end up in the upper slot of the furnace, not the lower one.

Variant 2:
1. Place a hopper on a furnace
2. Put some smelt-able stuff (in my case nether rack) in the hopper
3. The hopper places the smelt-able stuff in the upper slot of the furnace (correct behavior)
4. Put some coal blocks in the hopper
Result: The hopper will hold on to the coal blocks and never put them in the furnace, because the upper slot of the furnace is full.