If you buy a RapidSSL Geotrust SSL certificate and simply install it you will get "Invalid issuer" or such message and the browsers won't let user without confirmation.
To install the certificate correctly you have to Install RapidSSL intermediate certificate chain.
How? It's very easy.
In the file where you keep the Certificate itself simply add this certificate cahins (https://knowledge.rapidssl.com/library/VERISIGN/ALL_OTHER/RapidSSL%20Intermediate/RapidSSL_CA_bundle.pem)