Seems I may be encountering a bug. Let me try to describe it first.
Say you sketch a rectangle. And then within this rectangle you draw more line segments inside to give it sections. Some of those lines will start at the perimeter of the rectangle and go inward. You extrude this. And then you find that in this extrusion, axially, along the side, there is a vertical line that is not the vertex of an inside or outside corner, but just a line going from one end of the extrusion to the other. This line appears at the intersection of a single interior line with a perimeter line.
There are times that I will draw rather complex shapes to create an object. In the designs I do, the lines are always parallel, perpendicular, or diagonal, always at a 0º, 45º, or 90º angle. Never anything strange, and the lines always meet one another to create complete faces capable of being independently extruded.
You can see this in the pictures I’ve attached. I have a sketch, with a lot of stuff going on. I’ve extruded several different layers, incorporating different parts of the sketch on each one. I then union these together, and I’m left with lines on the side. Normally you’d think these might be evidence of an outside or inside corner, some shift in the surface. But these are on perfectly flat surfaces, extruded from a solid line that usually has a line at a right angle perfectly meeting it. There should be no lines.