In the past you were only allowed to have a single SSL certificate per instance of Apache. This changed with the advent of Server Name Indication (SNI) which allows multiple SSL certs on a single instance. This means that as part of the HTTP request the browser must send a request for the appropriate hostname in the request, so Apache knows which SSL cert to serve. This can sometime be problematic if your browser/OS does not support SNI. Most notably, no version of Windows XP + Internet Explorer support SNI. It may also be a problem in wildcard SSL installations where the hostname does not directly match the SSL cert.
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