Like the title says, I came upon this bug at least twice now. After some time (usually quite a lot of time), no matter on which map, invisible, impassable by any type of unit (ground, floating or flying), walls appear in most likely random places.
The first time it happened to me (during patch 0.8.6) I almost couldn't finish the match, because my melee units couldn't reach most of the enemy's base (god bless catapults), but this time the wall simply made it impossible to end the match in any way, because it blocked off the way to/from my base completely (Map being "The Cave"). I was also able to see many other units, such as (before the incident, very aggressive) neutral Reapers stuck in a small narrow area unable to move and not reacting to any nearby non-neutral troops.
The walls seem to be in rather unusual shapes, seeing how, during my first encounter with the bug, my army tried to form a long line to get to the enemies, which then just stood there in place because the linear way simply ended at some point. The walls, of course, are impassable for both my and any other units, whether they're neutral or the enemies'.
Sadly, I don't know any way of replicating the bug other than simply waiting for it to happen.