I'm baffled by what I'm seeing here...
NOTE: You will need to click on the image to see the line in question better.
It appears to be a blue line where I believe the edge of the layout border to be.
The 3D camera is positioned just above the layout in a rather 3d-person perspective and it flickers occasionally as I move my green circle character around and rotate the camera around behind it. I have to get the position just right to get the blue line to appear but I'm not sure what it is...
Interestingly, as you can see, it's appearing in front of those green towers (those are 3D cubes that I'm getting set in place). I was going to use these towers to block the blue line if it was, indeed, in the distance, but it still appears in front of the 3D blocks (visually more so than spacially).
When I had the 3D camera positioned higher, I never saw this happen. Also, I did recently double the size of the layout from 1500x1500 to 3000x3000.
Any idea what's causing this?