The pens don't seem to play very nicely with each other, or themselves if viewed from the wrong angle.  If you for instance draw a line with pen A, the draw a spiral around it with pen B, one of them will render in front of the other regardless of their position in space. The same thing happens with the start and end of each line. The end of the line will render in front of the start even if it is further back in space. You can see this pretty simply by drawing a horizontal spiral and walking around it.

Has anyone else ran into this?  It really limits the usefulness of the pens for making 3D works or art.