I find it hard to believe that manual build-sheet tampering would have worked at any time in the last 20 years. The build configuration is electronic, and everything on it gets distributed to the suppliers who get a build sequence of parts to supply, and those end up at the line just-in-time. The chap on the line just puts on whatever part is put in front of them - he doesn't get to pick and choose. The customers that supply interior components have the worst of this, because the build sequence could want (let's say) a grey headliner no sunroof, followed by another grey headliner with sunroof, followed by a black one for panoramic glass roof, followed by a beige one with sunroof, etc etc. They handle this by having racks of each configuration stored in an ASRS (automatic storage and retrieval system), and a robotic picker picks them out in the build sequence and puts them in a shipping rack in the right order. There's no way for anyone at that point in the process to know which vehicle is destined for an employee ... As an aside, I know someone who worked at a plant that supplied steering wheels to Ford Oakville. Multiply the possible combinations of colour, which model of car it's for, which trim level it has (the steering wheel buttons are different), and it ended up at several hundred possible steering wheel variations!
Monkeying around to some extent certainly isn't impossible ... someone on the line may have access to the parts for the next few cars in sequence, which might open up the possibility of swapping which part goes on this car with what was supposed to go on the next one, thus resulting in two messed-up units ...