A side-project I'm working on will alert you when SSL certificates are about to expire, preventing these things from happening. It'll also show you a overview of all the expiration dates of your certificates and domains, updated automatically.
It's not live yet, but if you're interested you can sign up for the launch mail here:
Such a thing would have been very helpful to me. My domain's SSL cert expired and I got a new one but didn't notice that my provider had shipped a different, newer intermediate certificate.
For three weeks I was showing a great big THIS CONNECTION IS UNTRUSTED screen to Firefox users and didn't know it.
It surprises me how common it is, which made me build Domainsquire to fix it. As today's event shows it even happens to the big boys, and this is a predictable, pre-emptively solvable downtime event, so it really shouldn't happen.
It's not live yet, but if you're interested you can sign up for the launch mail here:
http://www.domainsquire.com