I was able to reproduce the effect but I don't yet fully understand it.  Partly it appears when you have automatically-resizing `Graphics` objects contained in a `Grid` object that is wider than the Notebook window.  For example, using this code:

    blocks = Table[{Graphics[{Darker@Green, Rectangle[{0, 0}, {1000, 200}]}, 
         ImagePadding -> 0, PlotRangePadding -> 0]}, {i, 6}];
    
    myGrid = Grid[blocks, Spacings -> {0, 0}, Frame -> All];
    
    Style[myGrid, Magnification -> 3]

And in a Notebook window that is easily able to contain the Graphic I get a solid green area with only fine black dividers:

![enter image description here][1]

However, if I resize the Notebook window to a narrower-than-graphic size I see this:

![enter image description here][2]

However magnification also plays a role here.  Using a fixed `ImageSize`, which prevents resizing, combined with magnification shows a gap even when the Notebook is plenty wide to contain it:

    Style[
     myGrid /. g_Graphics :> Show[g, ImageSize -> 50],
     Magnification -> 4
    ]

![enter image description here][3]


  [1]: http://i.stack.imgur.com/ObTpO.png
  [2]: http://i.stack.imgur.com/nrm2N.png
  [3]: http://i.stack.imgur.com/WCnPr.png