Discovering OpenSCAD via its official tutorial:

> On the example above, the second cube sits exactly on top of the first cube. This is something that should be avoided as it’s not clear to OpenSCAD whether the two cubes form one object together. This issue can be easily solved by always maintaining a small overlap of about 0.001 - 0.002 between the corresponding objects.

This goes against the whole point of doing parametric design and should be logged as a bug. If the CAD software is unsure whether object spanning [0 to 1] and object spanning [1 to 2] are physically fused, that should be a bug. Mathematically they are fused.