Peterb makes an excellent case in point with the PSU example. It's worth saving that post for when someone queries whether spending £50 rather than £25 on a 500W PSU.
One of the reasons I went for the IP35 Pro over the IP35-E is it's solid state capacitors. I'm sure I wouldn't have issues with the cheaper board but I think it's worth spending the extra for the quality (within reason) especially as I am overclocking.
It's a while since I was actively involved in QA and batch testing of components for the satellite industry but iirc there are certain specifications that we would hold the manufacturer to with regard the number per 100 to be tested, either by us, them, or a 3rd party. If the components had passed this continually for a given period then the AQL is dropped. I'm sure British Standards or
There will be various standards required by various industries and as has been mentioned, critical parts for things like aircraft require higher standards. The thing is it's not just batch testing that is required but a whole Quality Management System that has to be in place. Some companies ask suppliers to be ISO 9000 compliant etc. Big companies will have produced their own internal documentation that will then be audited to comply with EU/US legislation.
QA can be a minefield and I've got a story or two. Anyway off to watch England v France.