How to fix SSL errors in Mac OS X browsers

So you know about SSL (or rather TLS) and you prefer things secure, so you request and pay for an officially signed certificate and configure your Apache to use it. The next days you’re feeling very Kevin Mitnicky, until some nitwit on Twitter trashes you for the ugly error-message he sees when trying to visit your supposedly “secure” site that is. What’s up with that?
Well, chances are that your disgruntled visitor was using a browser you didn’t test on, like Chrome on Mac for example? Because there is a small issue you have to take into account when “doing https”; both Chrome and Safari (but not Firefox) on Mac use OS X’s keychain, which does not have some of the intermediate certificates needed to establish the trust relationship between your signed certificate and the certificate authority’s root certificate.
As you can’t expect Apple to add intermediate certificates to their keychain by default (which Firefox does a pretty good job though) and you can’t ask all your OS X users to add the intermediate certificate by hand either,  you’ll have to solve this yourself. A good thing Apache can help you in that department with it’s SSLCertificateChainFile directive, which

sets the optional all-in-one file where you can assemble the certificates of Certification Authorities (CA) which form the certificate chain of the server certificate. This starts with the issuing CA certificate of the server certificate and can range up to the root CA certificate.

If there’s only one intermediate certificate missing between your’s and the CA’s, you can export it in good old Firefox (as a pem-file), place it in the same directory as the actual certificate and use SSLCertificateChainFile to tell Apache where to find it and that should solve the nasty errors those Twittering Mac-heads get.