Yeah Shell for thickening surfaces can get pretty easily confused if you have multiple surfaces joined together unless they are all very smooth to each other where they touch.

The easiest way to guarantee that you've got a smooth piece is to have just 1 single surface.

When there is a sharp corner the offsetting engine does not know how to extend the surfaces so that things meet up properly, but this is an area that is being worked on in the geometry library though.

- Michael