I have the same issue on my iPad. There are two furnaces that I can neither use nor destroy. When I touch them I am kicked out of the game. I got rid of all other furnaces in the area, and removed all the blocks surrounding both furnaces so that nothing is touching them, but still cannot destroy them. These furnaces are in a straight line with 4 blocks of space between them. I am playing in survival mode on the peaceful setting, so I do not have the ingredients to make TNT. I am playing version 0.6.1 alpha and I guess this is the iPad 4 (it is not called that at the Apple store but is the most recent full sized iPad.) I never had this problem before I created these two furnaces.
No. I have a furnace I had to wall up because I could not use or destroy it. I tried after every upgrade. Now, with 0.7.4, I was able to destroy it so I would say the issue is resolved.
Thanks for the large chests, btw. 🙂