I learned a quick, silly lesson today. I run some servers that tunnel using openvpn to facilitate our single sign on. I've migrated one before, and at the time, I foolishly did not create a new certificate/key pair for the new server and re-used the old one. During the point of the migration where I had both servers online at once, the two openvpn clients kept fighting - one connected, the other disconnected, and so forth, until I figured it out.
This time - I did things right - I generated a new certificate. HOWEVER, The subjects of the certificates were still the same, so they were being assigned the same IP address. This caused basically the same situation. Fortunately this time I was a bit quicker to realize. Make sure you differentiate your subject names when using openvpn. In general, you would already be doing this, but in the case of moving a server hosting a given domain from one box to another, since the domain name being served is the same there is an inclination to just type the same domain name in...don't do it. Make sure it's unique. Thanks for the forum post which lead to my answer, Jan Just Keijser!
I'm not sure if just the OU or CN must be different, or if both should be different. I erred on the side of caution and made sure both were distinct. I falsely assumed only the certificate itself had to be unique, but that's not the case.
This is a self-reminder blog post / post of shame. DOH!