It all depends on the part and why it was rejected....some simply get rejected and scrapped, where others can be fixed and reintroduced on the line. Each station on the line keeps track of EACH part in the progression and can again reject the part (or assembly) at any given time. It can never be forced through production.

Now that we have global suppliers, each and every part is barcoded so if a part/assembly in Mexico get rejected that is intended for another assembly in the US then production might be halted until the process is fixed. All the lines are managed by a central database and can track quality in real time. This data then is incorporated in the warranty department so if there are a lot of warranty issues coming in once the vehicles are on the road we can literally track WHY it failed and then work to fix those issues. To prove this, you may get a recall on a vehicle and in that notice it will say 'recall for vehicles built during X time'....your recall will fix the problem they already found and fixed/updated in the assembly line

What ive seem, depends on the line, and what item.

Some or most items could de-route to a QA, Repair, check-up station that has manual controls to see if its still withing the standards to be still OK even if robots, cameras and what not said it was not, and on other places, they just plain up trash the items (or recycle where possible)