While testing is an essential part of an upgrade process, it is important to understand that testing cannot be perfect. There are many reasons why an organization undergoing a conscientious effort to test their system prior to an upgrade may not be able to identify an issue that will arise in the production environment.
In most cases, mirroring a production environment exactly is not realistic. Without the same production workload and other activities that occur in a production environment, the best an organization can do is match as many of the important factors between environments as possible.
Read the Testing an OnBase Solution guide available on Community for information on best practices and recommendations for testing.