A meridian bug
Mapping code has a habit of going quietly wrong where a line crosses a discontinuity: the dateline, a zone boundary, or a meridian you chose as the edge of a projection. The figure is the whole of the original post that survived.
One path is drawn twice. For most of its length the two versions sit on top of each other and look brown. Along a short stretch they part: red one way, green the other. That is the bug. Somewhere in the transform, a segment is being clipped or unwrapped differently from its neighbour.
The original write-up of the cause went with the database. The picture is still a useful test: if your line style can produce this, the geometry is not as continuous as it looks.