Twice now I've seen the Blizzard "force a character off a mountain" effect try to move a player into a hex that is already occupied (and thus the effect fails to resolve), but then a character try to enter the mountain hex where the other character should no longer be.. and the game permanently locks up (it gets stuck on the character combat animation). I'm assuming that the actual bug is with the force movement effect on Blizzard sometimes selecting the wrong hex to move the player to (i.e. it selects an invalid destination).