[s0rce]

In medical device manufacturing you have systems in place that your vendors have to disclose changes to their manufacturing process that hopefully can catch stuff like this before people die. I can see how minute stuff gets easily passed off as not an important change.

[duskwuff]

Especially if the real change is a couple levels separated from the problem. For instance, I can imagine a situation where the manufacturer of that "special cloth" didn't even change anything themselves, but their lubricant supplier silently changed the formula of their sewing machine oil. (Or maybe even that one of the suppliers to the lubricant company changed something - it's turtles all the way down.)

[s0rce]

Yes, you would also audit the quality system for your suppliers to confirm they are sufficiently controlling for upstream changes. In theory you can have all your ducks in a row.

[shmeeed]

"In theory" is doing a lot of heavy lifting there. ;)

Depending on the product and quantity, you can factor your purchase price level times 2-10 for every level of sub- and sub-sub-supplier you want to have audited to your "wacky spec" - which may even still sound kinda reasonable, until you realize your attack surface is basically fractal to the n-th degree. The amount of process steps and auxiliaries used in manufacturing is absolutely staggering.

Edit: I need to add this depends a lot on the sector. There's useful certificates for a lot of industries, if you choose to believe them.