A friend had an SSL scare on public WiFi while in a coffee shop today. Her browser was warning her that every SSL certificate was invalid (except for
*.google.com). Eventually it stopped, and she overheard others in the coffee shop commenting that their tablet was finally able to connect (it was previously refusing).
I'm not sure, but this could be a man in the middle attack on the WiFi, in which the attacker (somehow) had a valid Google certificate and provided DNS records to point at their own machine.
In this scenario the browser is perfectly content to allow you to connect to this spoofed service. If you are not extremely familiar with SSL certificate authorities, a good way to assert a cert is not a forgery is to compare it to a known-good copy of the certificate. If the signatures match, then you are good to go.
But where can you get a known-good copy?