yeah,
The problem was that I placed the
DP near the LandRaider and rolled for scatter, this placed him ontop of the land raider in difficult terrain where he slided along and off.
Does the fact that the model can't stay there (without blue
tac!) make it Impassable?

In the game we turned the tank back to the correct way up and placed the
DP who found grip on the heavy bolters and he took his dangerous terrain test... 1min later the
DP made a run move off the LandRaider...
In Ians example.
If the BloodThirster is placed next to the fence(/trees/ruins) but then scatters 3" onto the top of the fence(/trees/ruins) but can't be balanced there does that make it a impassable mishap? I would find that a bit harsh! They are not impassable terrain.

Also I can see a situation happen when the bloodthirster needs to be placed on the fence to get his assault.
0"----------------11"-Fence-12"-----------18"
Because he can't be placed on the difficult terrain Fence;
Should his movement be reduced to 11" and deny the assault?
or
Should the player claim WMR to enable the assault?
PAnic...