Same with my granddad, he had to work in the Opel factories. Their gig was to make cars that would function when tested, but that would fail within a couple of days of delivery to the field.

So, whenever they could they would structurally weaken some element deep in the engine just enough to make it fail.

This is a lot more difficult than it sounds, it is a study in planned obsolescence, because if you make it fail to soon you get a bullet to the back there is not a lot of room for error.

Finally they settled on making a fairly small cut in the wall of a piston, apparently that was good enough to survive a test or two but would fail quickly enough under actual use.

First I am sorry to hear about your grandmother.

Second, how far you take the instructions in the booklet obviously depends on the work you do, and the higher up you are, the easier it is to screw up something without being found out.

You've been voted up for a post that is ambiguous in its asertion. What did your grandmother do? How old was she at the time?

I'm not sure what's ambiguous. Certainly I'm generalizing ("anyone") from an anecdote; I'm not trying to make a rock-solid historical claim.

I think most of her factory work was making shells. She was born in 1921; this would presumably been in 1943-45 or so.

What I meant say was it isn't clear to me if she was a worker or ran the factory.

Oh! Just a worker.