I don't like bushido's ghost idea, Minecraft can be startling enough without ghosts coming out of a wall while you're mining.
loving imagine that, a foggy cubist ghost pops out of the wall, goes "WOOOOooo", haunts you, floats away, and you walk into some lava because of the haunting status effect
but you don't remember the burning-to-death part because YOU'VE ALREADY FAINTED FROM FRIIIIIIIIIIGHT.