What type of guarantee does the certificate issuer offer?

A diamond certificate is considered an art.

It's the expert opinion of an individual graders of a certain diamond. 

Individual values and opinions are unique from one graders to another graders.

Diamond laboratories such as GIA and AGSL do issue certificates for diamonds, validating the certain attributes that the diamonds do have. 

There is no guarantee whatsoever from this company. These gemologists are hired by big companies such as GIA to help them grade thousands of diamonds. Their opinions are individually based and therefore they cannot be held liable for any different values and opinions. These gemologists are considered to be the highest authorities when it comes to appraising diamonds.

However they are not guaranteed to be 100% fault free, therefore no diamond certificate issuer is going to offer any guarantee. Unless a huge error has been made, there is no guarantee.