Ok, I'll my anecdotal comment to the stack... I work on large global identity management systems. These systems tend to be connected to many other systems in m

Ok, I'll my anecdotal comment to the stack... I work on large global identity management systems. These systems tend to be connected to many other systems in many datacentres and offices. These all have firewalls on the network connections and/or boxes. The systems need to exchange data using a myriad of protocols and authentication mechanisms. No business is prepared to replicate this in Dev, or even Test. The best you can do is 'best efforts' and test outside of Prod the stuff you can, and by documenting heavily the Prod topology go into testing in Prod with good knowledge of where it might break, and have a break-fix window during deployment where you can hopefully resolve any blockers you hit. If you can't fix the release during the window, the whole system gets rolled back and you try again in a few more weeks having performed a retrospective wrap up on the failed deployment.