No easy way to reproduce this but the game will just randomly freeze and you need to go back to dashboard and quit the game and reload. This is only just started to happen in 1.6.0.8 as the game was fine in previous betas with no freezing. It has happened in the overworld and the nether and seems to be more likely to occur if you are traveling around rather than staying within the already loaded chunks. In one occurrence it froze shortly after i had re-spawned from dying whilst trying to fight the Wither and when I reloaded the game the items i had picked up after my death just before it froze had disappeared and so had the Wither, but the items i had not picked up were still on the ground. Not too fussed with the 2nd part as seems related to the freezing rather than another actual bug.