Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
I expected to put coal in, but it wouldn't allow me to switch from the smelt slot to the fuel slot. I couldn't even take the iron ore out
What actually happened was...:
Describe what happened here
I couldn't get anything out of it, so I broke it and got everything back, but that means no iron for the buckets in a new game
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... Enter furnace
3. ...place smelting item in first slot
4. ...done