That's a good example. I've come across that problem lots of times myself.
I've found a small rectangular fill with a solid white background and no contour lines works fairly well as an alternative, but you do have to pull the edges back so they don't overlap the main lines either side.
If we were working purely in a 2d environment this wouldn't even be a problem, just a quick trim and the rogue line would be gone. But because we are working in a 3d program, little things like this take far too long to sort out, and they don't appear correctly if you save your plans to DWG format for consultants.
I don't know what the solution is, but ArchiCAD needs a better way of dealing with little problem areas like this. Faking stuff with patches, or little white lines and fills just feels wrong for 2010!
Mini rant over!