I have been running a server on snapshot and been updating with the updates, so today I just updated to 15w44a and I got some XP and then in the anvil I tried to combine my Diamond Sword that had Sharpness II, Unbreaking II, and Looting II with my other Diamond Sword that had Sharpness II and Unbreaking II. When I tried to grab that new sword out of the final slot it just didn't give me and forced me out of the GUI even though the anvil didn't break, it just got a little cracked.