Volumetrics mysterious issue

After rendering a complex image with volume scattering by splitting the view and rendering to a renderfarm (Sheep it) I got some nasty seams between the tiles :

http://i.imgur.com/UVLGkaHm.jpg

The alpha is intact though. Only the color does that.

The view is split by zooming the camera and animating the shift X and Y (using this addon).

What could be the cause for that?

I never used SheepIt, but I’ve seen similar errors when using that render farm. I suspect some render nodes are being setup with wrong settings.

After re-rendering the same job multiple times, this issue never happened again. Maybe something was fixed. What caused this is still a mystery for me.