Someone please notify a higher up about this. Sure, it isn't exactly top priority... but here it is: http://gyazo.com/4549169f6bc487a165e0b4053f851c04
This happened to all 4 corners. The creeper, for whatever reason, does not want to cover up the border (corner) of the map.
Just thought I should report :)
There is creeper on those cells. It renders as a triangle because two outside adjacent cells don't contain creeper. If there was a unit that took a single cell it would receive damage if it landed in any of the four corner cells. In other words this is working as designed.
Those beveled edges, tho.
It's one of those things that once noticed, you can't un-notice it.