Greetings, when i put bones and sap to furnace and then put any kind of fuel via shift+click it disappear(
i don’t know any other combinations but with that for glue it’s happening.
Destroy the furnace. There is 3rd hidden slot at the moment and your fuel went to this slot. Move fuel manually next time. 
2 Likes
This has been fixed in the next release.
13 Likes
I feel like this will break my love of the game (not that my self-destroyed home did that well enough). The hidden furnace slots is almost a Boundless Meme 
@james Another annoying furnace glitch, is that they keep stopping for no reason. I load the sucker up with a fresh stack of fuel, and 600 iron, then when I come check on it, its stopped at 17 items. This has been a problem for me for months, not just the last update. I’ve played with diff numbers of diff items, All my furnaces do this for no apparent reason.
It usually happens when you switch alts after starting the furnace. I learned to stay with the same alt until smelting is done and do other things, but obviously its not ideal - not always want to stay with that alt longer than to put fuel and materials inside furnace.
nevermind nevermind
Little does everyone know I’ve been hiding secret items in that hidden slot on hundreds of furnaces on every planet 
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I did try that, and appreciate the help. It still happens to me though. Its random and not consistent though, so hard to track.