The "units leaving the table are destroyed" GW principle is from fleeing troops leaving the board. GW hasn't to my knowledge, any firm principle about this in any of their systems?

Anyway, I agree that treating it as impassable terrain (except where mentioned, like in the planetfall rules) is the fairest way.

As long as the ruling is consistent, and exists, that's all that matters IMO. FrogBear gave an opinion, and frankly, it's one that hurts his armies more than any other (few in our group besides him play with Banelords or Lords of Battles which scatter 3d6cm, and Warhounds only scatter d6cm which would be easy to avoid off-tables with*). Are there any other units that have a random movement?

* Also, it has to be the first wound, AND it has to be a single wound, or there's no difference between auto-destroyed and impassable.

Morgan Vening