The following happens only in the blast furnace and smoker. The regular furnace works as expected.
If the furnace is in this state and you click the iron ingot recipe again, the furnace changes to this state:
One iron ore was added as expected, but the fuel was removed and smelt progress was lost.
In the case of clicking a different recipe, losing progress is expected, but fuel is also removed.
The output is also deleted (regardless of whether you click the same recipe or a different one), see