I used a lava bucket as fuel and removed the empty bucket before anything completed smelting. The flame indicator didn't start to deplete until the last item. This persisted through restarts.

What I expected to happen was...:
I thought the flame indicator would deplete gradually as the fuel was used up.

What actually happened was...:
It did eventually begin to deplete, but not until it was one smelt from being used up

Steps to Reproduce:
Smelt something with a lava bucket and watch the fuel indicator. I don't know about other fuel sources, but this happened with several lava buckets. Might be worth mentioning that the furnaces were placed when the world was Pre-Release 1