I thought the problem was being caused by the way you have outlined the ribbon.
My suggestion was to start and end your ribbons outline path OFF the image area, and have it draw completely outside the image area before switching sides. this will get rid of the weird edge effects caused by +antialias option.
Code: Select all
ribbon='M -2,15 L 10,15 C 27,1 77,24 85,8 L 101,8 L101,60 L 75,60 C 50,68 20,58 15,70 L-2,70 Z'
For more detains of the path construct see IM Examples, Draw, SVG Path Specification
http://www.imagemagick.org/Usage/draw/#paths
However even with that 'fix' the two pixels you are complaining about remains. regardless of the antialias setting. As such it looks like their is some bug being found at just those two specific points.
If you take your original code and 'turn on' the stroke outline even that outline does not color those two specific pixels!
Something is definitely screwy.