(All files needed to repro this issue are available here.)

The following code shows a preview but fails to render for me (see bad.scad):

union()
for (z = [0, 1])
translate([0, 0, z])
import("arduino-modified.stl");

It gives the error:

ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr: e_below != SHalfedge_handle() File: /usr/include/CGAL/Nef_3/SNC_FM_decorator.h Line: 418

The file arduino-modified.stl is a file that I exported from OpenSCAD itself.

Curiously, I do not get this error and the render works when I swap the .stl file for Arduino.ipt.stl, which was exported from Autodesk Inventor (see ok.scad).

System info

$ openscad --version
OpenSCAD version 2021.01
$ cat /etc/lsb-release 
DISTRIB_ID="Arch"
DISTRIB_RELEASE="rolling"
DISTRIB_DESCRIPTION="Arch Linux"

Notes

I think this may be related to #3841, but I find that issue a little hard to read and I thought it was worthwhile to upload a clean repro.

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