I wonder if that's what I ran into last night.  I had a pure melee champion in a battle who was unable to move, and without any ranged or spell abilities he just had to pass.  He was on the edge of the tactical map just like your spider is pictured, but I didn't notice whether a grid border was missing.

I've actually seen my champions appear unable to move even though the mouse pointer shows they should be able to. There were no highlighted squares showing their move radius. However, if there was a monster within what their radius should be, they were able to move to it and attack it if I right clicked the monster, but could not move if I right clicked an open square.

I've seen so many invalid starting positions in tactical maps that I've just given up on caring about them. And the tactical map with the giant statue/monument in the center has all the squares it's in as passable, so it looks like units are hiding inside the damn thing.

I've seen a lot of maps that randomly have a small 1 square bump (You couldn't even call it a hill, just rolling ground) in the middle of them that is, for some reason, impassable terrain.