I design in OpenSCAD (a programmer’s CAD tool where you write code and “run” it to make your model as opposed to interactively building). When it comes time for mass production you need an industry-standard “.STEP” file format, which OpenSCAD doesn’t support, but another tool called FreeCAD does. The FreeCAD’s importer works on OpenSCAD models, but it was written in 2013 so programming features added to OpenSCAD after that aren’t supported.
In particular this includes polyhedrons with faces, which was added in March 2015. This post will hopefully help out anyone who was as lost as I was in trying to understand why the FreeCAD importer isn’t behaving as expected. I will expand as I find more issues.