SSL Certificates Throwing Errors Randomly

SSL Certificates Throwing Errors Randomly

I purchased a cheap SSL certificate from NameCheap (GeoTrust RapidSSL, to
be precise), and many users are complaining that they are getting an
"Untrusted SSL certificate" error when visiting the page. I've gotten
compalints from the following enviornments:
Windows 7 with IE9
Windows 7 with latest Chrome
Windows Vista with latest Chrome
Windows 7 with latest Firefox
Windows XP with IE8
However, many users with those configurations experience no problems. I
personally use OS X and Windows 7 with Chrome Canary and the latest
Firefox (respectively) and have never seen an error.
What could be causing these seemingly inconsistent SSL warnings? I was
under the impression that the RapidSSL product was valid in 99+% of
browsers, but I constantly hear about invalid certificates from many
users.
I am using Nginx 1.4.1 with the following configuration:
listen 443 ssl;
listen [::]:443 ssl default ipv6only=on;
ssl on;
ssl_certificate /path/to/cert.crt
ssl_certificate_key /path/to/cert.key;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers RC4:HIGH:!aNULL:!MD5;
ssl_prefer_server_ciphers on;
Note: I have never heard about an iOS or OS X device throwing the error.