After getting burned by the pattern making tools in Inkscape (remember the houndstooth umbrella), I decided to consult the manual. Turns out, patterns always create seams. Something about anti-aliasing.
It's really disappointing to say the least. I created the above image as an experiment in repeating lines (gotta have this stuff down to do tartan right). It's got ugly seams all over the place. More experiments down the road I guess.