For some of my site visitors, the SSL certificate is failing. Whatever tests I do on various browsers for me the SSL certificate is valid.
I can't think of how to test this on client side, and to identify the problem.
How would you do this?
One client gets: fatal certificate unknown
While RouMao's answer is mostly correct, he has missed what is (IME) the most common problem with SSL certificates - the certificate you are using requires an interim certificate from the CA which you have not included in your certificate chain. Most CAs provide an online tool for analysing the certificate - try the one located here.
Also, is there any correlation with which browser being used? Notably, Chrome does not handle SSL v2 by default