Sometimes when using Quad Chamfer you will get a result like below.

The easiest way to fix this is to:

• Add an edit poly modifier on top of the quad chamfer modifier;
• Collapse the edge between the two vertices causing abnormalities (overlap);
• Then delete the interior edge that forms 2 triangles to form a quad.

A benefit of this approach is that you can edit the chamfer further prior to smoothing if you need to.