I noticed that when i stitched some of my HAED in 10 by 10 blocks, it caused faint lines to appear between the edge of the block and the rest of the stitching. Hopefully you can see what i mean in this picture, in the red circled bit you should be able to see faint vertical lines.

I have heard several other people complain of the same problem, especially those who stitch their whole piece in 10 by 10 blocks, or between pages. Rather than being caused by uneven tension, it appears to be simply because of the way the stitches are formed on the back of the fabric. Stitching in blocks causes an empty line to form on the back of your work, a line where there are no threads crossing from one side of the square to the other. This means that the stitches on the left of this line are being pulled to the left, and the stitches on the right are being pulled to the right, leaving a slight gap in the middle.
I have heard that the lines are erased with washing and ironing the project at the end, but personally i think i will just try to avoid them in the first place by not stitching in blocks.