Via Tyler Cowen, this comes from a Microsoft Research paper about why ordinary people are probably justified in ignoring most security advice on the internet:
Browser vendors have invested considerable effort in making it harder to ignore certificate errors. In Firefox version 3, when encountering an expired, invalid or self-signed certificate the user sees an interrupt page explaining that the SSL connection failed. If he chooses to add an exception he sees another interrupt page with more warnings and a choice to add an exception or “get me out of here.” If he elects (again) to add an exception he must click to get the certificate, view the certificate, and then add the exception. Internet Explorer 8 is somewhat less intrusive, but the procedure also seems designed to suggest that adding exceptions is very risky. Is it? Ironically, one place a user will almost certainly never see a certificate error is on a phishing or malware hosting site. That is, using certificates is almost unknown among the reported phishing sites in PhishTank. The rare cases that employ certificates use valid ones. The same is true of sites that host malicious content. Attackers wisely calculate that it is far better to go without a certificate than risk the warning. In fact, as far as we can determine, there is no evidence of a single user being saved from harm by a certificate error, anywhere, ever.
I’ve long wondered about those certificate errors I get from time to time, but apparently they’re just that: errors. Now I know I can just ignore them and still sleep soundly at night.