Factorio Forums

It is possible to infinitely generate certain intermediate products with a finite amount of ingredients in a machine that has a productivity of >100%.
By removing a machine, or simply resetting the recipe of the machine, after a productivity proc has generated an item before the actual recipe has generated an item, it is possible to keep the generated item while also recouping the initial materials for the recipe. This can be done indefinitely, and notably with high quality items like legendary circuits.

Here are a few Screenshots regarding the process:

The machine in question The inventory / machine before shenanigans. The inventory / machine during shenanigans. Here there has already been a productivity proc while the original recipe has not yet run its course. The inventory / machine after shenanigans. The recipe of the machine has been reset, resulting it the generated item as well as the initial ingredients being placed into the player's inventory. Since this process requires removing a machine or resetting its recipe, which is not possible during crafting, it cannot currently be automated without mods to my knowledge. Though with the use of worker robots, some circuitry and a power switch, I have been able to automate it to a fascinating degree.
I do not know whether this constitutes intended behaviour, but since it feels a bit like item duping, I though I'd let y'all know.

Welcome on forums. This is a duplicate of 120100. TL;DR: we know but there are no nice solution and it is not really an issue because it needs player interaction so it does not scale well.