Imagine you are drawing a map of county borders. You are given a set of polygons, one for each boundary, and you draw each polygon.
In places where two counties share a border you just end up drawing the border twice. In the absence of partial transparency effects, and with a solid pen, this is no problem.
But, on maps, borders of this kind are customarily shown by dash-dotted lines. In this case, situations like the one depicted below can happen:
Notice how the dash pattern, which normally is dash-dot-dot, gets screwed up where the two areas share a border. In this case, it happened to become a longdash-dot pattern, but in general it could do anything from coincidentally looking normal to creating a solid line.
How does/should map rendering software prevent artifacts of this kind from occuring?
The artifact is due the fact that the piece of border is drawn twice. Instead of trying to supress such artifacts, you could try to not draw border sections twice, by keeping a list of segments already drawn in memory, and if you encounter a stretch that’s already drawn, you don’t draw it again.