Here you can see I am creating inserters, and I've got 4 production modules in there. Inserters is not an intermediate product, and if I try to re-assign one of the production modules with another production module, it tells me "Productivity module is usable only on intermediate products" but it won't remove the existing incorrect module.
How to exploit:
- Create a blueprint similar to below, ensuring that production modules is part of your blueprint.
- Place the blueprint anywhere. It will always build the buildings first, and then populate it with the production modules after (placing by proxy)
- Before your bots place the production modules, set your production type to be anything you want. When the bots come to place the modules, it won't reject them for being the wrong module type.
