Another possible problem overlap base of sphere with extruded box at base.

[rluiten]

It doesn't get this one right with one of three different results

Changing to rect([-10, -10.1], 20, 2); has no issues.
So I suspect that bottom of box matching bottom of sphere is related.

import { extrude, plane, rect, sketch, sphere } from 'fluidcad/core';

sphere(10);

const p = plane('yz', { offset: -10 });
sketch(p, () => {
  rect([-10, -10], 20, 2);
});
extrude(20);

Ive been starting code in my working folder with code . to see outcomes or reload window inside VSCode can switch between different output

Outcome 1.
No bottom on box. most common oddness

Outcome 2.
VSCode report error "FluidCAD: unwind" maybe 1 in 5 or 6 times

Outcome 3.
Just edges of the objects, this one happens infrequently maybe 1 in 15+ times.

[marwndev]

The inconsistent error messages are caused by FluidCAD crashing because it fails to mesh the shape.
The real issue is the sphere touching the bottom face of the extrusion. I'm still researching if this is a limitation in the OCC kernel itself.

[rluiten]

I can fully appreciate how alignment can complicate things.
I've bumped into interesting effects of aligned things in a number of other rendering and modelling software over many years so it's not that surprising.