I built a model and it previewed correctly. When rendering for STL export (F6), I got an assertion error:
ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr: e->incident_sface() != SFace_const_handle() File: /mxe/usr/x86_64-w64-mingw32.static.posix/include/CGAL/Nef_S2/SM_const_decorator.h Line: 329
Most of the model rendered correctly, but one component disappeared.
Moving the object didn't change the problem, but isolating just the disappearing part gave me a correct render of that part.