In Peaceful Mode, some blocks will instantly break, if a mob is de-spawned.  If the block is broken with a pickaxe, no problem.  I was collecting torches with torches in my hand, accidently clicked a mob-containing block instead of the torch, it did the whole instant break with the mod de-spawn poof of smoke, then I lost the ability to open chests, furnaces, and so on and when I tried to save the game, it froze.  I had to force the game to shut down through task manager.  The world successfully reopened to a recent autosave point, prior to the issue.