Its a problem with Smooth Shading, you need to adjust what faces are in what smoothing groups to fix it. Play with it for a while and you\'ll figure it out.
Sometimes \"turning\" the faces 90 degrees helps... like delete the two faces and make two new faces to replace them except in on the other diagonal of the quad... that may be hard to understand, but basically, if the quad dips in, it can cause that effect, if you change the faces so the quad doesn\'t dip in, that effect will often go away.
