A mint, unused stamp is offered along with a PF certificate. The stamp has a faint but unmistakable hand-written annotation on the reverse. The certificate does not mention the annotation in the "we are of the opinion" statement.
Is this indicative of a potential valuation problem? Does it void the certificate? Any other issues?
A lot will depend on the date on the certificate. A cert represents a snapshot in time regarding the stamp. Anything could have happened to the stamp since that date. This is why it is often wise to get a new certification on expensive stamps with certs older than 10 or 20 years.
It is possible that the notation was there when the cert was issued, but its presence was not considered worth noting at the time. This is often true on older PF certs, which also tended to lack any comments regarding the presence or absence of hinging.