One of the SCEA book gives the following situation and asks how safe this is to trust the cert:
You find a certificate on an unknown Web site. The certificate signature validates the certificate as issued by a certificate authority using the key installed in the browser you copied from a friend.
Although sounds unsafe, it may very well yield a true certificate. Here is my reasoning, Since we CAN validate the cert. signature using the public key installed in the browser, that means we can use the public key in the browser to decrypt the cert. from this unknown web site. For this cert to be fake, the hacker has to both hack into the unknown web site to place their certificate there (signed by hacker's private key), AND ALSO hack into the site where my friend download his browser from, to place a compromised browser with their public key. This is not likely a situation for hacker to succeed as he has to hack into the exact two locations.