Most documents that cause trouble are genuine. They were issued to a real company, for a different product, and the scope is the part nobody reads.
A certificate gets checked once, usually by looking at a PDF. The PDF is the least reliable part of the document, because a PDF proves only that somebody can operate a printer.
Gate 1 is the one everyone does: does the number resolve. Gate 2 is where genuine documents fail — a real ISO certificate issued to a real company, for a scope that does not include what you are buying. Gate 3 catches the quiet case: valid when you asked for it, expired before you shipped.
Three of these caught buyers repeatedly. ISO 9001 certifies that a quality management system exists and is followed — it says nothing about whether the specific product is good. CE for many product directives permits the manufacturer to self-declare, so no notified body was involved and no independent test exists unless you ask for one. FDA registration is a facility listing; it is not an approval of any product, and buyers regularly treat the number as one.
A certificate is evidence about a system, or about a sample at a moment. It is never evidence about the unit in your container. The document tells you the factory has been organised to produce consistently; it does not tell you that it did, on your order, last week.
That gap is what an inspection closes, and it is the reason a certificate and an inspection are not substitutes for one another.
None of this means certificates are worthless. It means they answer a narrower question than the one buyers ask them. The narrow question is worth answering properly — three gates, ten minutes, no cost.
An audit checks the number against the issuing body and the scope against what you are actually buying.
Send a supplier to check