When removing items from the source side of the furnace the game will crash:

If you have placed multiple items in the top of the furnace and multiple coal in the bottom the game will crash about 60% of the time when you try to remove something. Either trying to remove something from the top or the coal from the bottom can replicate the result.

For example:

1) Place 64 cobblestone in the top of the furnace
2) Place 10 coal in the bottom
3) Let at least 1 piece of stone get created
4) Remove the cobblestone (might cause the crash)
5) Remove the coal (might cause the crash)
6) Add more cobblestone (might cause the crash)
7) Add more coal (might cause the crash)
8) Repeat 4 to 7 and it will crash in a little while

I saw this was on old problem in .7 but it happens on iPad 2, iPad 3 and iPad Air 2 in .10.4

All progress in the game will be lost. If you have played for an hour, all progress since the last time you "Quit to title" will be lost.

Workaround: Destroy the furnace