Meteors piling up at build limit will actually go through the border onto another planet- and the top layer isn't minable.

Screenshot taken with debug mode control F. As you can see, the meteors have piled up so high that they are in the undestroyable BORDER region of the build limit.

This only happens with sections where the neutronium border is too thin. However, that doesn't explain how the meteorites pile up to the other planet.