STEPS TO REPRODUCE
1. load http://www.whatwg.org/issues/data.html
2. zoom in on the top left
just the main line
If it's the fault of a bug in my script I apologise in advance. But I don't think it is.
also visible on the blue line where the main line jumps up for a bit.
Slightly more minimal testcase at http://nerget.com/bugs/bug20174.html
The issue is the 'dots' for somereason we occasionally get subpixel lines being drawn back to the origin. I suspect this is a CG-ism, but it'd be good to know if it occurs in the gtk/qt ports.